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Q:

My SQL statement SELECT AVG(SALARY) FROM EMP yields inaccurate results. Why?

Answer

Because SALARY is not declared to have NULLs and the employees for whom the salary is not known are also counted.


 

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Subject: Mainframe Interview

Q:

File contains 5 records say D,E,F,C,A-I want to copy D and A records only by using file-aid?

Answer

Go to file-aid copy option.Enter the from dataset to be copied and to dataset to which you want to copy.go to options. Enter 0 at Initial records to skip field.Then enter 3 at the no of records to skip field and 1 to no of records to select.press enter and PF3.

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Subject: Mainframe Interview

Q:

What is the self-referencing constraint?

Answer

A31. The self-referencing constraint limits in a single table the changes to a primary key that the related foreign key defines. The foreign key in a self referencing table must specify the DELETE CASCADE rule.

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Subject: Mainframe Interview

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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Subject: Mainframe Interview

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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Subject: Mainframe Interview

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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Subject: Mainframe Interview

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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Subject: Mainframe Interview