join_use_nulls

 :) select a.a, b.a from (select 1 as a) a left join (select 2 as a) b using a

SELECT
    a.a,
    b.a
FROM
(
    SELECT 1 AS a
) AS a
LEFT JOIN
(
    SELECT 2 AS a
) AS b USING (a)

Query id: df34015c-d440-410f-8b68-724419e47685

┌─a─┬─b.a─┐
│ 1 │   0 │
└───┴─────┘

1 rows in set. Elapsed: 0.236 sec. 

 :) set join_use_nulls=1

SET join_use_nulls = 1

Query id: 455cf9f4-1cad-4d0c-a743-2e036e1a1ae8

Ok.

0 rows in set. Elapsed: 0.193 sec. 

:) select a.a, b.a from (select 1 as a) a left join (select 2 as a) b using a

SELECT
    a.a,
    b.a
FROM
(
    SELECT 1 AS a
) AS a
LEFT JOIN
(
    SELECT 2 AS a
) AS b USING (a)

Query id: 6138dfbe-1a75-4319-813e-7068bb0ea1de

┌─a─┬──b.a─┐
│ 1 │ ᴺᵁᴸᴸ │
└───┴──────┘

1 rows in set. Elapsed: 0.199 sec.