[Solved] MySQL: NOT IN with sub select not working as expected?

Question

I will make the assumption that there is at least one record in sales_flat_order that satisfies condition status != 'holded' and whose customer_email is NULL.

(NOT) IN is notoriously tricky with NULLs, here is an example.

Consider the following query:

SELECT 1 WHERE 1 NOT IN (SELECT 2 UNION ALL SELECT 3)

This yields a record with value 1, as expected.

However if you change that to:

SELECT 1 WHERE 1 NOT IN (SELECT 2 UNION ALL SELECT NULL)

Then the query produces an empty result set. This is a well-known problem with (NOT) IN. For this reason, you should generally avoid this syntax, and use (NOT) EXISTS instead. The above query could be rewritten as:

SELECT 1 a
FROM (SELECT 1 a) t1
WHERE NOT EXISTS (
    SELECT 1
    FROM (SELECT 2 a UNION ALL SELECT NULL) t2
    WHERE t1.a = t2.a
)

Demo on DB Fiddle

For your query:

SELECT customer_email 
FROM sales_flat_order s
WHERE NOT EXISTS (
    SELECT 1
    FROM sales_flat_order s1
    WHERE s1.customer_email = s.customer_email AND s.status != 'holded'
);

Accepted Answer