= all Means Equal to Every Value

The = all operator means that for a row to satisfy the outer query the value in the column that introduces the subquery must be the same as each value in the list of values returned by the subquery.

For example, the following query identifies the authors who live in the same city at the postal code:

select au_fname, au_lname, city
from authors
where city = all
     (select city
     from authors
     where postalcode like "946%")