[mysql] How to test an SQL Update statement before running it?

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?

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.

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.

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.

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'

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

One more option is to ask MySQL for the query plan. This tells you two things:

• Whether there are any syntax errors in the query, if so the query plan command itself will fail
• How MySQL is planning to execute the query, e.g. what indexes it will use

In MySQL and most SQL databases the query plan command is describe, so you would do:

describe update ...;

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.

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.