INTERVIEW QUESTIONS MAINFRAME DB2 DETAILS

Question: 21) Why SELECT * is not preferred in embedded SQL programs?

Answer: For three reasons:

If the table structure is changed ( a field is added ), the program will have to be modified

Program might retrieve the columns which it might not use, leading on I/O over head.

The chance of an index only scan is lost.

What are correlated subqueries? -

A subquery in which the inner ( nested ) query refers back to the table in the outer query. Correlated subqueries must be evaluated for each qualified row of the outer query that is referred to.

For three reasons:

If the table structure is changed ( a field is added ), the program will have to be modified

Program might retrieve the columns which it might not use, leading on I/O over head.

The chance of an index only scan is lost.

What are correlated subqueries? -

A subquery in which the inner ( nested ) query refers back to the table in the outer query. Correlated subqueries must be evaluated for each qualified row of the outer query that is referred to. Source: CoolInterview.com

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