EXISTS does not try to compare on values, so it has no comparison operators.

It is common to use SELECT * in an EXISTS subselect, because the columns values do not matter.
EXISTS is simply testing for the existence of any rows, so format the subselect any way you like.
It does not matter how many columns or rows are returned by the subselect — one or a thousand.

If the subselect returns at least one row, EXISTS is true and NOT EXISTS is false.

If the subselect returns no rows, EXISTS is false and NOT EXISTS is true.

If the subselect returns a row that is null, EXISTS is true and NOT EXISTS is false.

> While EXISTS can have a simple subselect, it is normally associated with a Correlated Subselect.
See correlated subselect.