MySQL - UPDATE query based on SELECT Query

Question

I need to check (from the same table) if there is an association between two events based on date-time.

One set of data will contain the ending date-time of certain events and the other set of data will contain the starting date-time for other events.

If the first event completes before the second event then I would like to link them up.

What I have so far is:

SELECT name as name_A, date-time as end_DTS, id as id_A 
FROM tableA WHERE criteria = 1


SELECT name as name_B, date-time as start_DTS, id as id_B 
FROM tableA WHERE criteria = 2

Then I join them:

SELECT name_A, name_B, id_A, id_B, 
if(start_DTS > end_DTS,'VALID','') as validation_check
FROM tableA
LEFT JOIN tableB ON name_A = name_B

Can I then, based on my validation_check field, run a UPDATE query with the SELECT nested?

Answer 1

You can actually do this one of two ways:

MySQL update join syntax:

UPDATE tableA a
INNER JOIN tableB b ON a.name_a = b.name_b
SET validation_check = if(start_dts > end_dts, 'VALID', '')
-- where clause can go here

ANSI SQL syntax:

UPDATE tableA SET validation_check = 
    (SELECT if(start_DTS > end_DTS, 'VALID', '') AS validation_check
        FROM tableA
        INNER JOIN tableB ON name_A = name_B
        WHERE id_A = tableA.id_A)

Pick whichever one seems most natural to you.