Just found out how to do a LEFT JOIN on combined tables with UNION. It's a bit tricky, but the point is that the result of a UNION is another table... See below:

SELECT `ID`, "whatever columns" FROM (
SELECT "whatever" FROM t1
UNION DISTINCT
SELECT "whatever" FROM t2
) AS newName1
LEFT JOIN
(
SELECT `ID` FROM t3
UNION DISTINCT
SELECT `ID` FROM t4
) AS newName2
ON `newName1`.`ID` = `newName2`.`ID` WHERE `newName2`.`ID` IS NOT NULL

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