CODEWITHSUNDEEP
mysql
uzioriluzan
uzioriluzan

Reputation: 41

MySQL - Update the same column twice

I need to update a column in a mysql table. If I do it in two steps as below, is the column "col" updated twice in the disk ?

update table SET col=3*col, col=col+2;

or is it written only once as in :

update table SET col=3*col+2;

Thanks

Upvotes: 0

Views: 1738

Answers (3)

Wrikken
Wrikken

Reputation: 70490

See: http://dev.mysql.com/doc/refman/5.0/en/update.html

...
Single-table UPDATE assignments are generally evaluated from left to right. For multiple-table updates, there is no guarantee that assignments are carried out in any particular order.
...

I assume the column on disk is only updated once (row written), as it has to validate all assignments / fields in an update, it wouldn't do to write one field and then find out there is a unique key violation.

Upvotes: 3

Salil
Salil

Reputation: 47482

update table SET col=3*col+2;

this is better. JUST FOR RECORD where table is your tableName and not keyword

Upvotes: 3

D'Arcy Rittich
D'Arcy Rittich

Reputation: 171421

Both of the updates will run.

Upvotes: 1

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