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Q:

What are constraints? Mention the different types of constraints?

Answer

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


Types of Constraints: NOT NULL, CHEQUE, UNIQUE, PRIMARY KEY, FOREIGN KEY.

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Subject: SQL

Q:

What are defaults? Is there a column to which a default can’t be bound?

Answer

A default is a value which will be used by a column, if no value is supplied to that column while inserting data.


Yes, IDENTITY and TIMESTAMP columns are free from the bounds of default.

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Subject: SQL

Q:

What are user defined data-types and when do you use them?

Answer

User defined data-types in SQL is an extension to the base SQL server data-types by provision of a descriptive name.


We use them to format the database and segregate specific repetitions of rows and columns in the database.

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Subject: SQL

Q:

State one similarity and difference between WHERE Clause and HAVING Clause?

Answer

Similarity: Both WHERE and HAVING Clause filters out records based on one or more conditions.


Difference: WHERE Clause can only be applied to a static non-aggregated column whereas HAVING is needed for aggregated columns.

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Subject: SQL

Q:

How can you generate debugging output from PL/SQL?

Answer

Use the DBMS_OUTPUT package. Another possible method is to use the SHOW ERROR command.

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Subject: SQL

Q:

What are PL/SQL cursor exceptions?

Answer

Cursor_Already_Open,  Invalid_Cursor.

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Subject: SQL

Q:

What type of Joins have you used?

Answer

The knowledge of Joins is a MUST for every interviewee. Most SQL programmers have used inner join and  outer join[left/right]; but the catch point here is to also mention cross join and self-join.


 

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Subject: SQL

Q:

What is the difference between JOIN and UNION?

Answer

SQL JOIN allows access to records of other table based on the given conditions between two tables.


Ex: If we are provided with the department ID of each employee, then we can use the department ID of the employee table to merge with the department ID of the department table to access department names.


UNION operation allows addition of 2 similar data sets to create a resulting set that contains all the data from the source data sets.


Ex: If we are provided with 2 employee tables of the same structure, we can UNION them to create a singular result that will contain all employee from both the table.

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Subject: SQL