如何在 SET 中使用 MySQL CASE 而不会破坏它？

Question

I have many fields that I need to mass replace without destroying other data in that column.

If I run:

UPDATE ospos_items AS a
INNER JOIN ospos_items AS b ON a.item_id = b.item_id
SET a.custom7 = 'Detective Novels'
WHERE a.custom7 = 'DN'
AND a.custom3 = 'English Literature';

The Update is properly non-destructive. But if I run the following code in an effort to combine queries:

UPDATE ospos_items AS a
INNER JOIN ospos_items AS b ON a.item_id = b.item_id
SET a.custom7 = CASE
WHEN a.custom7 = 'DN' THEN 'Detective Novels'
WHEN a.custom7 = 'HF' THEN 'Historical Fiction'
WHEN a.custom7 = 'HUM' THEN 'Humor'
WHEN a.custom7 = 'NOV' THEN 'Novels'
WHEN a.custom7 = 'YA' THEN 'Young Adults'
END
WHERE a.custom3 IN('English Literature');

It sets all values in that column to NULL not found in the CASE. For example, some of the fields in that column have already been converted. How can I make my second query non-destructive to values not found in the CASE statement?

Answer 1

There are two solutions:

1. Add ELSE a.custom7 to the CASE expression. Then it will keep the same value if none of the WHEN conditions match.
2. Add AND a.custom7 IN ('DN', 'HF', 'HUM', 'NOV', 'YA') to the WHERE clause. Then it will only update the rows where one of the CASE expressions matches.

BTW, when you're testing the same column in every WHEN clause, you can use the simpler:

CASE a.custom7
WHEN 'DN' THEN 'Detective Novels'
WHEN 'HF' THEN 'Historical Fiction'
...
END

Answer 2

You need to add an ELSE clause to the CASE expression so that the value remains the same when it is not one of the ones you want to change:

UPDATE ospos_items AS a
INNER JOIN ospos_items AS b ON a.item_id = b.item_id
SET a.custom7 = CASE
WHEN a.custom7 = 'DN' THEN 'Detective Novels'
WHEN a.custom7 = 'HF' THEN 'Historical Fiction'
WHEN a.custom7 = 'HUM' THEN 'Humor'
WHEN a.custom7 = 'NOV' THEN 'Novels'
WHEN a.custom7 = 'YA' THEN 'Young Adults'
ELSE a.custom7
END
WHERE a.custom3 IN('English Literature');