SQL Interview Questions

What is ON DELETE CASCADE?

When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed

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.

What the difference between UNION and UNIONALL?

Union will remove the duplicate rows from the result set while Union all does'nt.

Which system table contains information on constraints on all the tables created ?
USER_CONSTRAINTS,
system table contains information on constraints on all the tables created

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.

What is the difference between SQL and SQL Server ?

SQLServer is an RDBMS just like oracle,DB2 from Microsoft
whereas
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.

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)

What is database?
A database is a collection of data that is organized so that itscontents can easily be accessed, managed and updated. open this url : http://www.webopedia.com/TERM/d/database.html

How can i hide a particular table name of our schema?
you can hide the table name by creating synonyms.

e.g) you can create a synonym y for table x

create synonym y for x;

What is difference between DBMS and RDBMS?
The main difference of DBMS & RDBMS is

RDBMS have Normalization. Normalization means to refining the redundant and maintain the stablization.
the DBMS hasn't normalization concept.

What are the advantages and disadvantages of primary key and foreign key in SQL?

Primary key

Advantages

1) It is a unique key on which all the other candidate keys are functionally dependent

Disadvantage

1) There can be more than one keys on which all the other attributes are dependent on.

Foreign Key

Advantage

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?
datediff

for Eg: select datediff (dd,'2-06-2007','7-06-2007')

output is 5


What is denormalization and when would you go for it?

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

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 nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.