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
How many times does a correlated subquery conceptually execute?
Answer: Once per outer-query row
A correlated subquery is re-evaluated for each row processed by the outer query.
Which returns rows less than AT LEAST ONE value from the subquery?
Answer: < ANY
< ANY is true if the value is less than at least one returned value.
Can a subquery appear in the HAVING clause?
Answer: Yes, to compare against aggregated results
Subqueries are valid in HAVING to filter groups based on computed or external values.
What is the result type expected by an IN subquery?
Answer: A single column of values
An IN subquery should return one column whose values are compared for membership.
Which often performs better than a correlated subquery for the same logic?
Answer: A JOIN
Rewriting a correlated subquery as a JOIN frequently lets the optimizer run it more efficiently.
What does = ALL require for the comparison to be TRUE with multiple distinct values?
Answer: It cannot be TRUE since one value cannot equal several different ones
= ALL is TRUE only if the value equals every returned value, impossible when they differ.
A subquery feeding an INSERT ... SELECT statement does what?
Answer: Supplies the rows to insert
INSERT ... SELECT uses the subquery's result set as the rows to be inserted.