Saturday, December 22, 2012

What is normalization? Explain different levels of normalization?
Detaching and attaching databases,
creating INSERT scripts to generate data.
    * Transactional replication (with immediate updating subscribers, with queued updating subscribers)
    * Merge replication
Salary between 40000 and 55000 -- 7000 hike
Salary between 55000 and 65000 -- 9000 hike
CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000
WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000
WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000
[INTO new_table_]
FROM table_source
[WHERE search_condition]
[GROUP BY group_by__expression]
[HAVING search_condition]
[ORDER BY order__expression [ASC | DESC] ]

empid int,
mgrid int,
empname char(10)
INSERT emp SELECT 2,3,'Mohan'
INSERT emp SELECT 4,2,'Shridhar'
INSERT emp SELECT 5,2,'Sourabh'
FROM emp t1, emp t2
WHERE t1.mgrid = t2.empid
FROM emp t1
emp t2
t1.mgrid = t2.empid


Query:  Select A.firstname , B.firstname
from t_employee A, t_employee B
where A.supervisor_id = B.employee_id;
ACID - Atomicity, Consistency, Isolation and Durability, these are the unique entities of a transaction.
                SQL Interview Questions and Answers :
ex: create trigger after Insert on
system table contains information on constraints on all the tables created
Normalisation means refining the redundancy and maintain stablisation. there are four types of normalisation :
first normal forms, second normal forms, third normal forms and fourth Normal forms.
Select * from global_name;
This will give the datbase name which u r currently connected to.....
Structured Query Language (SQL), pronounced "sequel", is a language that provides an interface to relational database systems. It was developed by IBM in the 1970s for use in System R. SQL is a de facto standard, as well as an ISO and ANSI standard. SQL is used to perform various operations on RDBMS.
Pattern matching operator is LIKE and it has to used with two attributes
2) What are disadvantages in Oracle and MS Access?
3) What are feratures&advantages in Oracle and MS Access?
Oracle's features for distributed transactions, materialized views and replication are not available with MS Access. These features enable Oracle to efficiently store data for multinational companies across the globe. Also these features increase scalability of applications based on Oracle.
A database is a collection of data that is organized so that itscontents can easily be accessed, managed and updated. open this url :
Clustered Index:- A Clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table may have only one clustered index.Non-Clustered Index:- A Non-Clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows in the disk. The leaf nodes of a non-clustered index does not consists of the data pages. instead the leaf node contains index rows.
you can hide the table name by creating synonyms.
The main difference of DBMS & RDBMS is
the DBMS hasn't normalization concept.

Check out the article Q100139 from Microsoft knowledge base and of course, there's much more information available in the net. It will be a good idea to get a hold of any RDBMS fundamentals text book, especially the one by C. J. Date. Most of the times, it will be okay if you can explain till third normal form.

What is de-normalization and when would you go for it?

As the name indicates, de-normalization is the reverse process of normalization. It is the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced.

How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables?

One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table.

It will be a good idea to read up a database designing fundamentals text book.

What's the difference between a primary key and a unique key?

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a non-clustered index by default. Another major difference is that, primary key does not allow NULLs, but unique key allows one NULL only.

What are user defined data types and when you should go for them?

User defined data types let you extend the base SQL Server data types by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar(8). In this case you could create a user defined data type called Flight_num_type of varchar(8) and use it across all your tables.

See sp_addtype, sp_droptype in books online.

What is bit data type and what's the information that can be stored inside a bit column?

Bit data type is used to store Boolean information like 1 or 0 (true or false). Until SQL Server 6.5 bit data type could hold either a 1 or 0 and there was no support for NULL. But from SQL Server 7.0 onwards, bit data type can represent a third state, which is NULL.

Define candidate key, alternate key, composite key.

A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys.

A key formed by combining at least two or more columns is called composite key.

What are defaults? Is there a column to which a default cannot be bound?

A default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can't have defaults bound to them. See CREATE DEFAULT in books online.

What is a transaction and what are ACID properties?

A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction. For more information and explanation of these properties, see SQL Server books online or any RDBMS fundamentals text book.

Explain different isolation levels

An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed, Repeatable Read, Serializable. See SQL Server books online for an explanation of the isolation levels. Be sure to read about SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation level at the connection level.

CREATE INDEX myIndex ON myTable (myColumn)

What type of Index will get created after executing the above statement?

Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.

What is the maximum size of a row?

8060 bytes. Do not be surprised with questions like 'What is the maximum number of columns per table'. Check out SQL Server books online for the page titled: "Maximum Capacity Specifications".

Explain Active/Active and Active/Passive cluster configurations

Hopefully you have experience setting up cluster servers. But if you do not, at least be familiar with the way clustering works and the two clustering configurations Active/Active and Active/Passive. SQL Server books online has enough information on this topic and there is a good white paper available on Microsoft site.

Explain the architecture of SQL Server

This is a very important question and you better be able to answer it if consider yourself a DBA. SQL Server books online is the best place to read about SQL Server architecture. Read up the chapter dedicated to SQL Server Architecture.

What is Lock Escalation?

Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it's dynamically managed by SQL Server.

What's the difference between DELETE TABLE and TRUNCATE TABLE commands?

DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it will not log the deletion of each row, instead it logs the de-allocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back.

Explain the storage models of OLAP

Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more information.

What are the new features introduced in SQL Server 2000 (or the latest release of SQL Server at the time of your interview)? What changed between the previous version of SQL Server and the current version?

This question is generally asked to see how current is your knowledge. Generally there is a section in the beginning of the books online titled "What's New", which has all such information. Of course, reading just that is not enough, you should have tried those things to better answer the questions. Also check out the section titled "Backward Compatibility" in books online which talks about the changes that have taken place in the new version.

What are constraints? Explain different types of constraints.

Constraints enable the RDBMS enforce the integrity of the database automatically, without needing you to create triggers, rule or defaults.


For an explanation of these constraints see books online for the pages titled: "Constraints" and "CREATE TABLE", "ALTER TABLE"

What is an index? What are the types of indexes? How many clustered indexes can be created on a table? I create a separate index on each column of a table. what are the advantages and disadvantages of this approach?

Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker.

Indexes are of two types. Clustered indexes and non-clustered indexes. When you create a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. Non-clustered indexes are stored as B-tree structures (so do clustered indexes), with the leaf level nodes having the index key and it's row locater. The row located could be the RID or the Clustered index key, depending up on the absence or presence of clustered index on the table.

If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same time, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.

What is RAID and what are different types of RAID configurations?

RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance to database servers. There are six RAID levels 0 through 5 offering different levels of performance, fault tolerance. MSDN has some information about RAID levels and for detailed information, check out the RAID advisory board's homepage

What are the steps you will take to improve performance of a poor performing query?

This is a very open ended question and there could be a lot of reasons behind the poor performance of a query. But some general issues that you could talk about would be: No indexes, table scans, missing or out of date statistics, blocking, excess recompilations of stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much normalization, excess usage of cursors and temporary tables.

Some of the tools/ways that help you troubleshooting performance problems are:

·       SQL Server Profiler,
·       Windows NT /2000 Performance monitor,
·       Graphical execution plan in Query Analyzer.
Download the white paper on performance tuning SQL Server from Microsoft web site.

What are the steps you will take, if you are tasked with securing an SQL Server?

Again this is another open ended question. Here are some things you could talk about: Preferring NT authentication, using server, database and application roles to control access to the data, securing the physical database files using NTFS permissions, using an unguessable SA password, restricting physical access to the SQL Server, renaming the Administrator account on the SQL Server computer, disabling the Guest account, enabling auditing, using multi-protocol encryption, setting up SSL, setting up firewalls, isolating SQL Server from the web server etc.

Read the white paper on SQL Server security from Microsoft website. Also check out My SQL Server security best practices

What is a deadlock and what is a live lock? How will you go about resolving deadlocks?

Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other's piece. Each process  would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user's process.

A livelock is one, where a  request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely.

Check out SET DEADLOCK_PRIORITY and "Minimizing Deadlocks"  in SQL Server books online. Also check out the article Q169960 from Microsoft knowledge base.

What is blocking and how would you troubleshoot it?

Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first.

Read up the following topics in SQL Server books online: Understanding and avoiding blocking, Coding efficient transactions.

Explain CREATE DATABASE syntax

Many of us are used to creating databases from the Enterprise Manager or by just issuing the command:


But what if you have to create a database with two file groups, one on drive C and the other on drive D with log on drive E with an initial size of 600 MB and with a growth factor of 15%? That's why being a DBA you should be familiar with the CREATE DATABASE syntax. Check out SQL Server books online for more information.

How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?

SQL Server can be started from command line, using the SQLSERVR.EXE. This EXE has some very important parameters with which a DBA should be familiar with. -m is used for starting SQL Server in single user mode and -f is used to start the SQL Server in minimal configuration mode. Check out SQL Server books online for more parameters and their explanations.

As a part of your job, what are the DBCC commands that you commonly use for database maintenance?


But there are a whole load of DBCC commands which are very useful for DBAs. Check out SQL Server books online for more information.

What are statistics, under what circumstances they go out of date, how do you update them?

Statistics determine the selectivity of the indexes. If an indexed column has unique values then the selectivity of that index is more, as opposed to an index with non-unique values. Query optimizer uses these indexes in determining whether to choose an index or not while executing a query.

Some situations under which you should update statistics:

1.  If there is significant change in the key values in the index
2.  If a large amount of data in an indexed column has been added, changed, or removed (that is, if the distribution of key values has changed), or the table has been truncated using the TRUNCATE TABLE statement and then repopulated
3.  Database is upgraded from a previous version
Look up SQL Server books online for the following commands:


What are the different ways of moving data/databases between servers and databases in SQL Server?

There are lots of options available, you have to choose your option depending upon your requirements. Some of the options you have are:


Explain different types of BACKUPs available in SQL Server? Given a particular scenario, how would you go about choosing a backup plan?

Types of backups you can create in SQL Sever 7.0+ are Full database backup, differential database backup, transaction log backup, filegroup backup. Check out the BACKUP and RESTORE commands in SQL Server books online. Be prepared to write the commands in your interview. Books online also has information on detailed backup/restore architecture and when one should go for a particular kind of backup.

What is database replication? What are the different types of replication you can set up in SQL Server?

Replication is the process of copying/moving data between databases on the same or different servers. SQL Server supports the following types of replication scenarios:

    * Snapshot replication

See SQL Server books online for in-depth coverage on replication. Be prepared to explain how different replication agents function, what are the main system tables used in replication etc.

How to determine the service pack currently installed on SQL Server?

The global variable @@Version stores the build number of the sqlservr.exe, which is used to determine the service pack installed. To know more about this process visit SQL Server service packs and versions.

What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?

Cursors allow row-by-row processing of the resultsets.

Types of cursors:


See books online for more information.

Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one round trip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors.

Most of the times, set based operations can be used instead of cursors. Here is an example:

If you have to give a flat hike to your employees using the following criteria:

Salary between 30000 and 40000 -- 5000 hike

In this situation many developers tend to use a cursor, determine each employee's salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:

UPDATE tbl_emp SET salary =

Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don't have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row.

Write down the general syntax for a SELECT statements covering all the options.

Here's the basic syntax: (Also checkout SELECT in books online for advanced syntax).

SELECT select_list

What is a join and explain different types of joins?

Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table.

Types of joins:


OUTER JOINs are further classified as


For more information see pages from books online titled: "Join Fundamentals" and "Using Joins".

Can you have a nested transaction?


What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?

An extended stored procedure is a function within a DLL (written in a programming language like C, C++ using Open Data Services (ODS) API) that can be called from T-SQL, just the way we call normal stored procedures using the EXEC statement. See books online to learn how to create extended stored procedures and how to add them to SQL Server.

Yes, you can instantiate a COM (written in languages like VB, VC++) object from T-SQL by using sp_OACreate stored procedure.

Also see books online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, sp_OADestroy.

What is the system function to get the current user's user id?

USER_ID(). Also check out other system functions like


What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?

Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table.

In SQL Server 6.5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 7.0 there's no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder

Triggers cannot be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined.

Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.

Till SQL Server 7.0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers.

Also check out books online for 'inserted table', 'deleted table' and COLUMNS_UPDATED()

There is a trigger defined for INSERT operations on a table, in an OLTP system. The trigger is written to instantiate a COM object and pass the newly inserted rows to it for some custom processing.

What do you think of this implementation? Can this be implemented better?

Instantiating COM objects is a time consuming process and since you are doing it from within a trigger, it slows down the data insertion process. Same is the case with sending emails from triggers. This scenario can be better implemented by logging all the necessary data into a separate table, and have a job which periodically checks this table and does the needful.

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What is a self join? Explain it with an example.

Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example: Employees table which contains rows for normal employees as well as managers. So, to find out the managers of all the employees, you need a self join.


INSERT emp SELECT 1,2,'Vyas'

SELECT t1.empname [Employee], t2.empname [Manager]

Here is an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)

SELECT t1.empname [Employee], COALESCE(t2.empname, 'No manager') [Manager]


Q1 - Write a query to find the total number of rows in a table
A1 - Select count(*) from t_employee;
Q2 - Write a query to eliminate duplicate records in the results of a table
A2 - Select distinct * from t_employee;
Q3 - Write a query to insert a record into a table
A3 - Insert into t_employee values ('empid35','Barack','Obama');
Q4 - Write a query to delete a record from a table
A4 - delete from t_employee where id='empid35';
Q5 - Write a query to display a row using index
A5 - For this, the indexed column of the table needs to be set as a parameter in the where clause
select * from t_employee where id='43';
Q6 - Write a query to fetch the highest record in a table, based on a record, say salary field in the t_salary table
A6 - Select max(salary) from t_salary;
Q7 - Write a query to fetch the first 3 characters of the field designation from the table t_employee
A7 - Select substr(designation,1,3) from t_employee; -- Note here that the substr function has been used.
Q8 - Write a query to concatenate two fields, say Designation and Department belonging to a table t_employee
Select Designation + ‘ ‘ + Department from t_employee;
Q9 -What is the difference between UNION and UNION ALL in SQL?
A9 - UNION is an SQL keyword used to merge the results of two or more tables using a Select statement, containing the same fields, with removed duplicate values. UNION ALL does the same, however it persists duplicate values.
Q10 - If there are 4 SQL Select statements joined using Union and Union All, how many times should a Union be used to remove duplicate rows?
A10 - One time.
Q11 - What is the difference between IN and BETWEEN, that are used inside a WHERE clause?
A11 - The BETWEEN clause is used to fetch a range of values, whereas the IN clause fetches data from a list of specified values.
Q12 - Explain the use of the ‘LIKE’ keyword used in the WHERE clause? Explain wildcard characters in SQL.
A12 - LIKE is used for partial string matches. The symbol ‘%’ ( for a string of any character ) and ‘_’ (for any single character ) are the two wild card characters used in SQL.
Q13 - What is the need to use a LIKE statement?
A13 - When a partial search is required in a scencario, where for instance, you need to find all employees with the last name having the sequence of characters "gat", then you may use the following query, to match a search criteria:
Select empid, firstname, lastname from t_employee where lastname like ‘%gats%’
This might search all employees with last name containing the character sequence 'gats' like Gates, Gatsby, Gatsburg, Sogatsky, etc.
% is used to represent remaining all characters in the name. This query fetches all records contains gats in the e middle of the string.
Q14 - Explain the use of the by GROUP BY and the HAVING clause.
A14 - The GROUP BY partitions the selected rows on the distinct values of the column on which the group by has been done. In tandem, the HAVING selects groups which match the criteria specified.
Q15 - In a table t_employee, the department column is nullable. Write a query to fetch employees which are not assigned a department yet.
A11.  Select empid, firstname, lastname from t_employee where department is null;
Q16 -What are the large objects supported by oracle and db2? What are the large objects supported in MS SQL?
A16 - In Oracle and DB2 BLOB , CLOB ( Binary Large Objects, Character Large Objects) are used.
In MS SQL - the data types are image and varbinary.
Q17 - Whats the capacity of the image data type in MS SQL?
A17 - Variable-length binary data with a maximum length of 2^31 - 1 (2,147,483,647) bytes.
Q18 - Whats the capacity of varbinary data type in MS SQL?
A18 - Variable-length binary data with a maximum length of 8,000 bytes.
Q19 - What’s the difference between a primary key and a unique key?
A19 - Both Primary key and Unique key enforce the uniqueness of a column on which they are defined. However, a Primary key does not allow nulls, whereas unique key allow nulls.
Q20 - What are the different types of joins in SQL?
Q21 - What is a Self join?
A21 - A join created by joining two or more instances of a same table.
Q22 - What is a transaction and ACID?
A22 - Transaction - A transaction is a logical unit of work. All steps must be committed or rolled back.
What is the difference between oracle,sql and sql server ?
·       Oracle is based on RDBMS.
·       SQL is Structured Query Language.
·       SQL Server is another tool for RDBMS provided by MicroSoft.
why you need indexing ? where that is stroed and what you mean by schema object? For what purpose we are using view?
We cant create an Index on Index.. Index is stoed in user_index table.Every object that has been created on Schema is Schema Object like Table,View etc.If we want to share the particular data to various users we have to use the virtual table for the Base table...So tht is a view.
indexing is used for faster search or to retrieve data faster from various table. Schema containing set of tables, basically schema means logical separation of the database. View is crated for faster retrieval of data. It's customized virtual table. we can create a single view of multiple tables. Only the drawback is..view needs to be get refreshed for retrieving updated data.
Difference between Store Procedure and Trigger?
·       we can call stored procedure explicitly.
·       but trigger is automatically invoked when the action defined in trigger is done.
·       this trigger invoked after we insert something on that table.
·       Stored procedure can't be inactive but trigger can be Inactive.
·       Triggers are used to initiate a particular activity after fulfilling certain condition.It need to define and can be enable and disable according to need.
What is the advantage to use trigger in your PL?
Triggers are fired implicitly on the tables/views on which they are created. There are various advantages of using a trigger. Some of them are:
·       Suppose we need to validate a DML statement(insert/Update/Delete) that modifies a table then we can write a trigger on the table that gets fired implicitly whenever DML statement is executed on that table.
·       Another reason of using triggers can be for automatic updation of one or more tables whenever a DML/DDL statement is executed for the table on which the trigger is created.
·       Triggers can be used to enforce constraints. For eg : Any insert/update/ Delete statements should not be allowed on a particular table after office hours. For enforcing this constraint Triggers should be used.
·       Triggers can be used to publish information about database events to subscribers. Database event can be a system event like Database startup or shutdown or it can be a user even like User loggin in or user logoff.
What the difference between UNION and UNIONALL?
Union will remove the duplicate rows from the result set while Union all does'nt.
What is the difference between TRUNCATE and DELETE commands?
Both will result in deleting all the rows in the table .TRUNCATE call cannot be rolled back as it is a DDL command and all memory space for that table is released back to the server. TRUNCATE is much faster.Whereas DELETE call is an DML command and can be rolled back.
Which system table contains information on constraints on all the tables created ?
Explain normalization ?
How to find out the database name from SQL*PLUS command prompt?
What is the difference between SQL and SQL Server ?
SQLServer is an RDBMS just like oracle,DB2 from Microsoft
What is diffrence between Co-related sub query and nested sub query?
Correlated subquery runs once for each row selected by the outer query. It contains a reference to a value from the row selected by the outer query.
Nested subquery runs only once for the entire nesting (outer) query. It does not contain any reference to the outer query row.
For example,
Correlated Subquery:
select e1.empname, e1.basicsal, e1.deptno from emp e1 where e1.basicsal = (select max(basicsal) from emp e2 where e2.deptno = e1.deptno)
Nested Subquery:
select empname, basicsal, deptno from emp where (deptno, basicsal) in (select deptno, max(basicsal) from emp group by deptno)
1. % and
2. _ ( underscore )
% means matches zero or more characters and under score means mathing exactly one character
1)What is difference between Oracle and MS Access?
What is database?
What is cluster.cluster index and non cluster index ?
How can i hide a particular table name of our schema?
e.g) you can create a synonym y for table x
create synonym y for x;
What is difference between DBMS and RDBMS?
RDBMS have Normalization. Normalization means to refining the redundant and maintain the stablization.
What are the advantages and disadvantages of primary key and foreign key in SQL?
Primary key
1) It is a unique key on which all the other candidate keys are functionally dependent
1) There can be more than one keys on which all the other attributes are dependent on.
Foreign Key
1)It allows refrencing another table using the primary key for the other table
Which date function is used to find the difference between two dates?
for Eg: select datediff (dd,'2-06-2007','7-06-2007')
output is 5
1. Q. What does SQL stand for? 
   A. Structured Query Language

   Q. Who was E. F. Codd?
   A. He was the original inventor of the relational model.

2. Q. How do you select all records from the table?
   A. Select * from table_name;

   Q. What do you understand by the term referential integrity?

3. Q. What is a join?
   A. Join is a process of retrieve pieces of data from different sets (tables) and returns them to the user or program as one “joined” collection of data.

4. Q. What kinds of joins do you know? Give examples.
   A. We have self join, outer joint (LEFT, RIGHT), , cross-join ( Cartesian product n*m rows returned)
outer joint
SELECT Employee.Name, Department. DeptName
FROM Employee, Department
WHERE Employee.Employee_ID = Department.Employee_ID;

SELECT * FROM table1, table2;

self join
SELECT | |’    ‘ | | e2.ename FROM emp e1, emp e2 WHERE e1. emp_no = e2.emp_no;

The following summarizes the result of the join operations:
 The result of T1 INNER JOIN T2 consists of their paired rows where the
join-condition is true.
 The result of T1 LEFT OUTER JOIN T2 consists of their paired rows where
the join-condition is true and, for each unpaired row of T1, the
concatenation of that row with the null row of T2. All columns derived
from T2 allow null values.
 The result of T1 RIGHT OUTER JOIN T2 consists of their paired rows
where the join-condition is true and, for each unpaired row of T2, the
concatenation of that row with the null row of T1. All columns derived
from T1 allow null values.
The result of T1 FULL OUTER JOIN T2 consists of their paired rows and,
for each unpaired row of T2, the concatenation of that row with the null
row of T1 and, for each unpaired row of T1, the concatenation of that row
with the null row of T2. All columns derived from T1 and T2 allow null

5. Q. How do you add record to a table?
   A. INSERT into table_name VALUES (‘ALEX’ , 33 , ‘M’);

6.  Q. How do you add a column to a table?
    A. ALTER TABLE Department
      ADD (AGE,  NUMBER);

7.  Q. How do you change value of the field?
    A. UPDATE EMP_table
       set  number = 200 where item_munber = ‘CD’;

update name_table set status = 'enable'  where phone = '4161112222';

update SERVICE_table set REQUEST_DATE = to_date ('2006-03-04 09:29', 'yyyy-mm-dd hh24:MI') where phone = '4161112222';

8.  Q. What does COMMIT do?
    A. Saving all changes made by DML statements

9.  Q. What is a primary key?
    A. The column  (columns) that has completely unique data throughout
the table is known as the primary key field.

10. Q. What are foreign keys?
    A. Foreign key field – is a field that links one table
to another table’s primary or foreign key.

11. Q. What is the main role of a primary key in a table?
    A. The main role of a primary key in a data table is to maintain the internal integrity of a data table.

12. Q. Can a table have more than one foreign key defined?
    A. A table can have any number of foreign keys defined. It can have only
       one primary key defined.

13. Q. List all the possible values that can be stored in a BOOLEAN data field.
     A. There are only two values that can be stored in a BOOLEAN data field:
         -1(true) and 0(false).

14  Q. What is the highest value that can be stored in a BYTE data field?
    A. The highest value that can be stored in a BYTE field is 255. or from -128
       to 127. Byte is a set of Bits that represent a single character.
        Usually there are 8 Bits in a Byte, sometimes more, depending on how
        the measurement is being made. Each Char requires one byte of memory
        and can have a value from 0 to 255 (or 0 to 11111111 in binary).

15.  Q. How many places to the right of the decimal can be stored in a
         CURRENCY data field?
     A. The CURRENCY data type can store up to four places to the right of the
        decimal. Any data beyond the fourth place will be truncated by Visual
        Basic without reporting an error.

16.  Q. What is a stored procedure?
     A. A procedure is a group of PL/SQL statements that can be called by
        a name. Procedures do not return values they perform tasks.

17.  Q. Describe how NULLs work in SQL?
     A. The NULL is how SQL handles missing values.
      Arifthmetic operation with NULL in SQL will return a NULL.

18.  Q. What is Normalization?
     A. The process of table design is called normalization.

19.   Q. What is referential integrity constraints?
      A. Referential integrity constraints are rules
       that are partnof the table in a database schema.

20.    Q. What is Trigger?
       A. Trigger will execute a block of procedural code
       against the database when a table event occurs.
       A2. A trigger defines a set of actions that are performed in response
       to an insert, update, or delete operation on a specified table. When 
       such an  SQL operation is executed, in this case the trigger has been

21.    Q. Which of the following WHERE clauses will return only rows
 that have a NULL in the PerDiemExpenses column?
A.      WHERE PerDiemExpenses <>
B.      WHERE PerDiemExpenses IS NULL
C.      WHERE PerDiemExpenses = NULL
D.      WHERE PerDiemExpenses NOT IN (*)

      A. B is correct � When searching for a NULL value in a column, you must
 use the  keyword IS. No quotes are required around the keyword NULL.

22.   Q. You issue the following query:SELECT FirstName FROM
StaffListWHERE FirstName LIKE'_A%'Which names would be
returned by this query? Choose all that apply.
A.      Allen
B.      CLARK
D.      David

      A. C is correct � Two wildcards are used with the LIKE operator.
The underscore (_) stands for any one character of any
case, and the percent sign (%) stands for any number of
characters of any case including none. Because this string
starts with an underscore rather than a percent sign, it won't
return Allen or Clark because they represent zero and two
characters before the "A". If the LIKE string had been "%A%",
both of these values would have been returned.
David was not returned because all non-wild card characters
are case sensitive. Therefore, only strings
with an uppercase "A" as their second letter are returned

23.   Q. Write a SQL SELECT query that only returns each city only once from Students table?
Do you need to order this list with an ORDER BY clause?

FROM Students;

The Distinct keyword automatically sorts all data
 in ascending order. However, if you want the data
 sorted in descending order, you have to use an ORDER BY clause

24.    Q. Write a SQL SELECT sample of the concatenation operator.

       A.  SELECT LastName ||',' || FirstName, City FROM Students;

25.    Q. How to rename column in the SQL SELECT query?

       A.  SELECT LastName ||',' || FirstName
          AS "Student Name", City AS "Home City"
           "FROM StudentsORDER BY "Student Name"

26.   Q. Write SQL SELECT example how you limiting the rows returned with a WHERE clause.

      A. SELECT InstructorID, Salary FROM Instructors
WHERE Salary > 5400 AND Salary < 6600;

27.   Q. Write SQL SELECT query that returns the first and
last name of each instructor, the Salary,
and gives each of them a number.

       A. SELECT FirstName, LastName, Salary,
ROWNUM FROM Instructors;

28.    Q. Which of the following functions can be used only with numeric values?
(Choose all that apply.)
A.      AVG
B.      MIN
C.      LENGTH
D.      SUM
E.      ROUND

       A. A and D � Only A and D are correct. The MIN function
works with any character, numeric, or date datatype.
The LENGTH function is a character function that returns
the number of letters in a character value. The ROUND
function works with both numeric and date values.

29. Q.  Which function do you use to remove all padded characters
to the right of a character value in a column with a char datatype?
A.      RTRIM
B.      RPAD
C.      TRIM

     A.  C � The TRIM function is used to remove padded spaces.
LTRIM and RTRIM functions were included in earlier versions
of Oracle, but Oracle 8i has replaced them with a single
TRIM function

30.  Q. Which statement do you use to eliminate padded spaces
 between the month and day values in a function TO_CHAR(SYSDATE,'Month, DD, YYYY') ?

     A. To remove padded spaces, you use the "fm"
prefix before the date element that contains the spaces.
     TO_CHAR(SYSDATE,'fmMonth DD, YYYY')

31. Q. Is the WHERE clause must appear always before the GROUP BY clause in SQL SELECT ?

    A. Yes.
The proper order for SQL SELECT
Only the SELECT and FROM clause are mandatory.

32. Q. How Oracle executes a statement with nested subqueries?

    A. When Oracle executes a statement with nested subqueries,
it always executes the innermost query first. This query passes its
results to the next query and so on until it reaches the outermost query.
It is the outermost query that returns a result set.

33. Q. Which operator do you use to return all of the rows
from one query except rows are returned in a second query?

    A. You use the MINUS operator to return all rows from one query except
where duplicate rows are found in a second query. The UNION operator
returns all rows from both queries minus duplicates. The UNION ALL operator
returns all rows from both queries including duplicates.
The INTERSECT operator returns only those rows that exist in both queries.

34. Q. How you will create a column alias? (Oracle 8i)

    A. The AS keyword is optional when specifying a column alias.
You must enclose the column alias in double quotes when the alias
contains a space or lowercase letters. If you specify an alias in l
owercase letters without double quotes, the alias will appear in uppercase.

35  Q. Which of the following statements are Data Manipulation Language commands?
A.      INSERT
B.      UPDATE
C.      GRANT
E.      CREATE

    A.  A and B � The INSERT and UPDATE statements are
Data Manipulation Language (DML) commands.
GRANT is a Data Control Language (DCL) command.
TRUNCATE and CREATE are Data Definition Language (DDL) commands

36. Question. What is Oracle locking?
    A. Oracle uses locking mechanisms to protect data from
being destroyed by concurrent transactions.

37. Question. What Oracle lock modes do you know?
     A.  Oracle has two lock modes: shared or exclusive.
Shared locks are set on database resources so that many transactions
 can access the resource.
Exclusive locks are set on resources that ensure
one transaction has exclusive access to the database resource

38.  Question. What is query optimization?
     A.  Query optimization is the part of the query
process in which the database system compares
different query strategies and chooses the one with
the least expected cost

39.  Question. What are the main components of Database management systems software.
     A. The database management system software includes
 components for storage management, concurrency control, transaction
processing, database manipulation interface, database definition interface,
and database control interface.

40.  Question. What are the main attributes of  database management system?
     A. A database management system is composed of five elements: computer hardware, software, data, people (users), and operations procedures.

41.  Question. What is transaction?
      A. A transaction is a collection of applications
code and database manipulation code bound into an indivisible unit of execution.
it consists from:

42.  Question.  What databases do you know?

43.  Question. Explain SQL SELECT example:
     select j.FILE_NUM
  from DB_name.job j, DB_name.address a
where j.JOB_TYPE ='C'
  AND j.OFFICE_ID = '101'
  AND a.OFFICE_ID = '101'
  AND a.FILE_NUM = j.FILE_NUM order by j.FILE_NUM;

    Answer: j and a aliases for table names. this is outer joint select statament from two tables.

44.  Q. Describe some Conversion Functions that you know

      A. TO_CHAR converts a number / date to a string.
         TO_DATE converts a string (representing a date) to a date.

         TO_NUMBER converts a character string containing digits to a numeric data type, it accepts one parameter which is a column value or a string literal

45.   Q.  Describe some Group Functions that you know

A. 1) The COUNT function tells you how many rows were in the result set.

    2) The AVG function tells you the average value of a numeric column.

    3) The MAX and MIN functions tell you the maximum and minimum value of a numeric column.

     4) The SUM function tells you the sum value of a numeric column.

46. Question. What does DML stand for? 
     A. DML is Data Manipulation Language statements. (SELECT)

47. Question. What does DDL stand for?
    A. DDL is Data Definition Language statements. (CREATE)

48.  Question. What does DCL stand for?

     A. DCL is Data Control Language statements. (COMMIT)

49.  Question: Describe SQL comments.

     A. SQL comments are introduced by two consecutive hyphens
     (--) and ended by the end of the line.

50.   Q. In what sequence SQL statement are processed?

      A. The clauses of the subselect are processed in the following sequence (DB2):
1. FROM clause
2. WHERE clause
3. GROUP BY clause
4. HAVING clause
5. SELECT clause
6. ORDER BY clause
7. FETCH FIRST clause

51.    Q. Describe TO_DATE function.

       A. The TO_DATE function returns a timestamp from a character string
that has been interpreted using a character template.
TO_DATE is a synonym for TIMESTAMP_FORMAT.

52.    Question:
 In the domain table we have status as a numeric value from 01 to 04 and we
 have text definition of these values in the design document.
 Write SQL query to see the result as a text definitions that is corresponded
 to these values. (DB2)
      A. select TB1.member_id, TB1.bu_id, TB1.program,  TB2.num,
        case TB1.status
                when '01' then 'Auto renew'
                when '02' then 'Expired'
                when '03' then 'Sold'
                when '04' then ‘Terminated’
        else TB_name.status
        from DB_name.TB_name1  TB1,
        DB_name.TB_name2 TB2
        TB1.program in ('com', 'org')
        and TB1.member_role  = '100'
        order by  TB1.member_id
        fetch first 30 rows only

53. Question:
What's the logical difference, if any, between the following SQL expressions?



 A. They're the same unless table T is empty, in which case the first yields
a one-column, one-row table containing a zero and the second yields a one-column,
one-row table "containing a null."

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