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MySQL: update a field only if condition is met

Posted by: admin November 1, 2017 Leave a comment

Questions:

Is it possible to do an UPDATE query in MySQL which updates field value only if certain condition is met? Something like this:

UPDATE test
SET
    CASE
        WHEN true
        THEN field = 1
    END
WHERE id = 123

In other words:

UPDATE test
SET
    something = 1,        /*field that always gets updated*/
    CASE
        WHEN true
        THEN field = 1    /*field that should only get updated when condition is met*/
    END
WHERE id = 123

What is the proper way to do this?

Answers:

Yes!

Here you have another example:

UPDATE prices
SET final_price= CASE
   WHEN currency=1 THEN 0.81*final_price
   ELSE final_price
END

This works because MySQL doesn’t update the row, if there is no change, as mentioned in docs:

If you set a column to the value it currently has, MySQL notices this
and does not update it.

Questions:
Answers:

Try this:

UPDATE test
SET
   field = 1
WHERE id = 123 and condition

Questions:
Answers:

Another solution which, in my opinion, is easier to read would be:

UPDATE test SET something = 1, field = IF(condition is true, 1, field) WHERE id = 123

What this does is set ‘field’ to 1 (like OP used as example) if the condition is met and use the current value of ‘field’ if not met. Using the previous value is the same as not changing, so there you go.

Questions:
Answers:

Another variation:

UPDATE test
SET field = IF ( {condition}, {new value}, field )
WHERE id = 123

This will update the field with {new value} only if {condition} is met