根据同一行中的另一个 DATETIME 值在 MYSQL 中设置 MONTH

Question

I have two date fields in each row of the same table. I'm trying to figure out how to update just the MONTH of one date based on whether it is greater or less than the other date.

Here's what I need to execute: IF the second date is GREATER THAN the first date, then set the MONTH of the first date equal to the MONTH of the second date.

So if the first date is '2021-03-16' and the second date is '2021-05-01', I want to update the first date to equal '2021-05-16'.

Or, for a less verbose description, how do I transform this...

Data set:

id date_1       date_2
 1 2021-03-16   2021-05-01
 2 2021-03-16   2021-02-01

...into this...

id date_1       date_2
 1 2021-05-16   2021-05-01
 2 2021-03-16   2021-02-01

Any suggestions on the correct method/syntax to do this would be greatly appreciated.

Answer 1

UPDATE table
SET first_date = CONCAT_WS('-', YEAR(first_date), 
                                MONTH(GREATEST(first_date, second_date)), 
                                DAY(first_date));

Answer 2

You can add to date_1 as many months as is the difference of the months in the 2 dates:

UPDATE tablename
SET date_1 = date_1 + INTERVAL MONTH(date_2) - MONTH(date_1) month
WHERE date_1 < date_2

See the demo .
Results:

id	date_1	date_2
1	2021-05-16	2021-05-01
2	2021-03-16	2021-02-01

Note, that your requirement may fail in case of dates like date_1 = '2021-03-31' and date_2 = '2021-04-30' because this '2021-04-31' is not a valid date.
But the above query will work by changing the day also.