Writing Subqueries Flashcards
7 cards from real SQL practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Writing Subqueries flashcards as text
What does NOT EXISTS return when its subquery yields no rows?
Answer: TRUE
NOT EXISTS is TRUE precisely when the subquery returns no rows.
Which comparison returns rows greater than EVERY value the subquery returns?
Answer: > ALL
> ALL is true only when the value exceeds every value returned by the subquery.
A NULL in a NOT IN subquery list can cause what problem?
Answer: The whole predicate may return no rows unexpectedly
NOT IN with a NULL in the list can yield UNKNOWN, filtering out rows you expected to keep.
Which is generally preferred for performance when checking existence of related rows?
Answer: EXISTS
EXISTS can stop at the first matching row, often outperforming counting approaches.
In SELECT salary, (SELECT AVG(salary) FROM emp) AS avg_sal, the inner query is a what?
Answer: Scalar subquery in the SELECT list
It returns one value placed in the SELECT list, making it a scalar subquery.
What must a subquery in the FROM clause always have?
Answer: An alias
Derived tables in FROM must be named with an alias in most SQL dialects.
Which keyword can replace IN for multi-row equality matching?
Answer: = ANY
= ANY is logically equivalent to IN, matching any value in the result set.