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Q:

What do you accomplish by GROUP BY ... HAVING clause?

Answer

GROUP BY partitions the selected rows on the distinct values of the column on which you group by. HAVING selects GROUPs which match the criteria specified

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Subject: Mainframe Interview

Q:

What is the difference between primary key & unique index ?

Answer

Primary key : a relational database constraint. Primary key consists of one or more columns that uniquely identify a row in the table. For a normalized relation, there is one designated primary key.


Unique index: a physical object that stores only unique values. There can be one or more unique indexes on a table.

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Q:

Why is free space left in KSDS Dataset?

Answer

While allocating KSDS Datasets, free space is declared at regular intervals at the time of initial loading. This is done because this free space is utilized for keeping the data arranged in sequence physically, even though inserted randomly.

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Q:

What is the need to code COMMITS in batch programs?

Answer

COMMIT statements are used to release locks which are required for that unit of work, and then permit a new unit of work. In other words, if COMMITS are not coded in the program and the program has been sent for execution, then while processing, in place of just going back to a few inserts since the nearest commit, the program has to go back to the inserts which were made during the entire run of the program. This extra process takes around twice or thrice the time taken normally.

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Q:

How can record locking be achieved in those DB2 versions which do not support it?

Answer

It can be done by setting the record length’s size to more than half of the page’s size.

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Q:

What is NOTCAT2?

Answer

It is an MVS message that indicates the presence of a duplicate catalogue in the database. This problem can be fixed by removing one of the entries and then adding a new one.

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Q:

Name and explain the type of locks?

Answer

There are three types of lock, namely:


Shared – This type of lock permits two or more programs to read from the locked space but does not allow them to change it.


Update – This lock is more lenient. It permits the program to read and change the locked space.


Exclusive – This lock restricts all users from accessing the locked space.

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Q:

What does a deadlock mean in DB2?

Answer

When two independent processes contend for the same resource or the resources reserved by one another, it is called a deadlock. -911 and -913 are the SQLcode for a deadlock.

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