How to test an SQL Update statement before running it?

Question

In some cases, running an UPDATE statement in production can save the day. However a borked update can be worse than the initial problem.

Short of using a test datab

Answer 1

In addition to using a transaction as Imad has said (which should be mandatory anyway) you can also do a sanity check which rows are affected by running a select using the same WHERE clause as the UPDATE.

So if you UPDATE is

UPDATE foo
  SET bar = 42
WHERE col1 = 1
  AND col2 = 'foobar';

The following will show you which rows will be updated:

SELECT *
FROM foo
WHERE col1 = 1
  AND col2 = 'foobar';

Answer 2

Not a direct answer, but I've seen many borked prod data situations that could have been avoided by typing the WHERE clause first! Sometimes a WHERE 1 = 0 can help with putting a working statement together safely too. And looking at an estimated execution plan, which will estimate rows affected, can be useful. Beyond that, in a transaction that you roll back as others have said.

Answer 3

I know this is a repeat of other answers, but it has some emotional support to take the extra step for testing update :D

For testing update, hash # is your friend.

If you have an update statement like:

UPDATE 
wp_history
SET history_by="admin"
WHERE
history_ip LIKE '123%'

You hash UPDATE and SET out for testing, then hash them back in:

SELECT * FROM
#UPDATE
wp_history
#SET history_by="admin"
WHERE
history_ip LIKE '123%'

It works for simple statements.

An additional practically mandatory solution is, to get a copy (backup duplicate), whenever using update on a production table. Phpmyadmin > operations > copy: table_yearmonthday. It just takes a few seconds for tables <=100M.