MySQL COUNT() and nulls

Question

Am I correct in saying:

COUNT(expr)
WHERE expr IS NOT *  

Will count only non nulls?

Will COUN

Answer 1

Correct. COUNT(*) is all rows in the table, COUNT(Expression) is where the expression is non-null only.

If all columns are NULL (which indicates you don't have a primary key, so this shouldn't happen in a normalized database) COUNT(*) still returns all of the rows inserted. Just don't do that.

You can think of the * symbol as meaning "in the table" and not "in any column".

This is covered in the MySQL Reference Manual.