IBM Certification SQL Database 2

Question 1

Which SQL isolation level prevents dirty reads but still allows non-repeatable reads?

Accepted Answer

READ COMMITTED

Answer

READ COMMITTED prevents dirty reads by only seeing committed data, but another transaction can modify rows between reads in the same transaction.

Question 2

In IBM Db2, what does the FETCH FIRST n ROWS ONLY clause do?

Accepted Answer

Limits the result set to the first n rows

Answer

FETCH FIRST n ROWS ONLY is Db2's syntax to restrict result set size, equivalent to LIMIT in other databases.

Question 3

What is a correlated subquery?

Accepted Answer

A subquery that references a column from the outer query

Answer

A correlated subquery references columns from the outer query and is re-executed for each row processed by the outer query.

Question 4

Which aggregate function returns the number of rows that have a non-NULL value in a specified column?

Accepted Answer

COUNT(column)

Answer

COUNT(column) counts only non-NULL values in the specified column, while COUNT(*) counts all rows including NULLs.

Question 5

What SQL keyword is used to remove duplicate rows from a result set?

Accepted Answer

DISTINCT

Answer

The DISTINCT keyword in a SELECT statement eliminates duplicate rows from the result set.

IBM Certification Practice Test