Trying to parse this sql

Question

I'm trying to understand what this SQL (from a MySQL installation) actually does:

IF(coalesce(a.entity_id, 0) != 0, 0, 1)

While I understand what the coalesce does I don't understand how the IF statement is modifying it.

Answer 1

I think:

coalesce(a.entity_id, 0) - return the first not null value,

if a.entity_id is not null you get 0 as a result of if, else 1.

a.entity_id = null => coalesce = 0 => if = 1

a.entity_id is not null => coalesce = a.entity_id => if = 0

Answer 2

coalesce returns the first NON-null argument. So if a.entity_id is null, coalesce will return 0. the containing if() then checks if the argument is not zero, and returns 0 or 1.

basically it's a convoluted way of writing a.entity_id IS NULL.