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 database, what are options to tell what an update statement will do before running it?

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

What about Transactions? They have the ROLLBACK-Feature.

@see https://dev.mysql.com/doc/refman/5.0/en/commit.html

For example:

START TRANSACTION;
SELECT * FROM nicetable WHERE somthing=1;
UPDATE nicetable SET nicefield='VALUE' WHERE somthing=1;
SELECT * FROM nicetable WHERE somthing=1; #check

COMMIT;
# or if you want to reset changes 
ROLLBACK;

SELECT * FROM nicetable WHERE somthing=1; #should be the old value

Answer on question from @rickozoe below:

In general these lines will not be executed as once. In PHP f.e. you would write something like that (perhaps a little bit cleaner, but wanted to answer quick ;-) ):

$MysqlConnection->query('START TRANSACTION;');
$erg = $MysqlConnection->query('UPDATE MyGuests SET lastname='Doe' WHERE id=2;');
if($erg)
    $MysqlConnection->query('COMMIT;');
else
    $MysqlConnection->query('ROLLBACK;');

Another way would be to use MySQL Variables (see https://dev.mysql.com/doc/refman/5.7/en/user-variables.html and https://stackoverflow.com/a/18499823/1416909 ):

# do some stuff that should be conditionally rollbacked later on

SET @v1 := UPDATE MyGuests SET lastname='Doe' WHERE id=2;
IF(v1 < 1) THEN
    ROLLBACK;
ELSE
    COMMIT;
END IF;

But I would suggest to use the language wrappers available in your favorite programming language.

Answer 3

Autocommit OFF ...

MySQL

set autocommit=0;

It sets the autommit off for the current session.

You execute your statement, see what it has changed, and then rollback if it's wrong or commit if it's what you expected !

EDIT: The benefit of using transactions instead of running select query is that you can check the resulting set easierly.

Answer 4

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.

Answer 5

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 6

Run select query on same table with all where conditions you are applying in update query.

Answer 7

In these cases that you want to test, it's a good idea to focus on only current column values and soon-to-be-updated column values.

Please take a look at the following code that I've written to update WHMCS prices:

# UPDATE tblinvoiceitems AS ii

SELECT                        ###  JUST
    ii.amount AS old_value,   ###  FOR
    h.amount AS new_value     ###  TESTING
FROM tblinvoiceitems AS ii    ###  PURPOSES.

JOIN tblhosting AS h ON ii.relid = h.id
JOIN tblinvoices AS i ON ii.invoiceid = i.id

WHERE ii.amount <> h.amount   ### Show only updatable rows

# SET ii.amount = h.amount

This way we clearly compare already existing values versus new values.

Answer 8

make a SELECT of it,

like if you got

UPDATE users SET id=0 WHERE name='jan'

convert it to

SELECT * FROM users WHERE name='jan'