mysql when sub-query return null

Question

I try to do the the following query with mysql ( Should add more conditions, but simplified it for the question , so the sub-query sometimes return null and sometimes return value, this is just for making the query shorter for the question):

SELECT COUNT(*)
FROM table
WHERE date = (SELECT MAX(date) FROM table)

My problem is that if the sub-query return null, my result will be 0, which is not the desired result. Since I can't do IS instead of = , I'm wondering if there is a simple solution.

Answer 1

MySQL provides a NULL safe equality comparison <=> (spaceship) operator.

I suspect that if you replace the = equality comparison operator with the NULL safe equality comparison operator, the query will return the results it looks like you are after.

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This expression:

  a <=> b

is basically shorthand equivalent for:

  a = b OR ( a IS NULL AND b IS NULL )

Answer 2

要计算所有具有null日期的行，这应该工作：

SELECT COUNT(*) FROM table WHERE date IS NULL;

Answer 3

This should not be a concern.

The subquery only returns NULL if there are no rows or if date is NULL in all rows.

The query will return 0 in this case. That makes sense to me. What would you want it to return?

Answer 4

You can make a union of both results and calculate the max:

SELECT MAX(datecount) FROM
(SELECT COUNT(*) AS datecount FROM table WHERE date IS NULL 
UNION ALL
SELECT COUNT(*) AS datecount FROM table WHERE date = (SELECT MAX(date) FROM table)) AS derivedtable