What is the difference between count(0), count(1).. and count(*) in mySQL/SQL?

Question

I was recently asked this question in an interview. I tried this in mySQL, and got the same results(final results). All gave the number of rows in that particular table. Can

Answer 1

Since the expression 1 is a constant expression, they should always produce the same result, but the implementations might differ as some RDBMS might check whether 1 IS NULL for every single row in the group. This is still being done by PostgreSQL 11.3 as I have shown in this article.

I've benchmarked queries on 1M rows doing the two types of count:

-- Faster
SELECT count(*) FROM t;

-- 10% slower on PostgreSQL 11.3
SELECT count(1) FROM t;

One reason why people might use the less intuitive COUNT(1) could be that historically, it was the other way round.