问题
Stack Overflow. PHP and SQL novice here.
As part of a multi-user private messaging system I've been trying to write to learn how to properly interact with a database through PDO, I have two separate queries that are executed when a user deletes a message, via a single function deleteMessage():
UPDATE messages SET s_deleted = 1 WHERE id = :id AND sender = :sender
UPDATE messages SET r_deleted = 1 WHERE id = :id AND recipient = :recipient
They work well enough to accomplish what I need but running both one after the other, as I am currently doing, doesn't strike me as particularly efficient.
I looked into CASE, but as far as I could understand it wasn't quite what I needed.
Is there a way to combine these two queries so that I'm not peppering my database with extraneous requests? Would I be better off splicing each query into a separate function, i.e., deleteMessageSender() and deleteMessageRecipient()?
回答1:
UPDATE messages
SET s_deleted = IF(sender = :sender, 1, s_deleted),
r_deleted = IF(recipient = :recipient, 1, r_deleted)
WHERE id = :id
As you can see, the "trick" is to simply set a column to its existing value when the criteria isn't met, so it only gets updated when necessary.
回答2:
UPDATE messages
SET s_deleted = IF(sender = :sender, 1, s_deleted),
r_deleted = IF (recipient = :recipient, 1, r_deleted)
WHERE id = :id AND
(sender = :sender OR recipient = :recipient);
来源：https://stackoverflow.com/questions/16763595/php-mysql-combine-update-queries