MySQL Where IS NOT ALL NULL? - mysql

Question

How can a query with ability to verify all selected records are not null?
For example with the following query,
verify all rows with field is_field_null is not NULL from returned level1?
SELECT * FROM (
SELECT tb_a.A, tb_b.is_field_null FROM tb_a, tb_b, tb_c
WHERE tb_a.a = "some" AND tb_a.b = "thing" AND tb_c.c = "else"
AND tb_a.b = tb_b.a
AND tb_c.b = tb_b.c
) AS level1
Suppose there are some rows with is_field_null in level1 are indeed NULL and the tables are
tb_a
a | b | A
------------------------------------------
"some" | "thing" | "XXX"
"some" | "thing" | "YYY"
tb_b
a | c | is_field_null
----------------------------------------------
"thing" | "else" | "I have things here"
"thing" | "else" | NULL
tb_c
b | c | mapper
----------------------------------------------
"else" | "else" | "ZZZ"
"else" | "else" | "KKK"
I have tried the following which it returns some row(s) with is_field_null is not null. E.g.
A | is_field_null
-----------------------------
"XXX" | "I have things here"
"YYY" | "I have things here"
SELECT * FROM (
SELECT tb_a.A, tb_b.is_field_null FROM tb_a, tb_b, tb_c
WHERE tb_a.a = "some" AND tb_a.b = "thing" AND tb_c.c = "else"
AND tb_a.b = tb_b.a
AND tb_c.b = tb_b.c
) AS level1
WHERE level1.is_field_null IS NOT NULL
I would expect an empty table. How can I do it?
E.g.
SELECT * FROM (
SELECT tb_a.A, tb_b.is_field_null FROM tb_a, tb_b, tb_c
WHERE tb_a.a = "some" AND tb_a.b = "thing" AND tb_c.c = "else"
AND tb_a.b = tb_b.a
AND tb_c.b = tb_b.c
) AS level1
WHERE level1.is_field_null IS NOT ALL NULL ??

Answer 1

If you are running MySQL 8.0, a simple method uses window functions:
SELECT *
FROM (
SELECT
tb_a.A,
tb_b.is_field_null,
MAX(tb_b.is_field_null IS NULL) has_null
FROM tb_a
INNER JOIN tb_b ON tb_a.b = tb_b.a
INNER JOIN tb_c ON tb_c.b = tb_b.c
WHERE
tb_a.a = 'some'
AND tb_a.b = 'thing'
AND tb_c.c = 'else'
) t
WHERE has_null = 0
Note that this uses standard, explicit joins rather than old-school, implicit joins - this ancient syntax should not be used in new code.
Also I would recommend single quotes instead of double quotes for literal strings (this is the MySQL syntax).