Databases · Middle · Question 2 of 75

There's an index on `(last_name, first_name)`. Which query can use it efficiently?

1Neither: a two-column index works only when both columns are in WHERE
2WHERE first_name = 'Ann'
✓Correct answer: WHERE last_name = 'Smith'

Explanation

A composite index is sorted by the first column, then by the second within it, like a phone book. It helps queries on a leading prefix of its columns; searching only by first_name usually can't use it well.

There's an index on `(last_name, first_name)`. Which query can use it efficiently?

Explanation

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There's an index on (last_name, first_name). Which query can use it efficiently?

Explanation

Further reading

Test yourself

There's an index on `(last_name, first_name)`. Which query can use it efficiently?