删除链接的MySQL表中的行并减少另一个表中的计数

Question

I am trying to create (java-based) messaging webapp. The messages are stored using MySQL using three tables:

• 'messages' : has two columns, an id primary key ('idmessage') and the message ('message').
• 'tags' : The user also inputs a tag associated with their message. This has two columns - the tag (which is also a primary key) and a count ('count').
• 'message_tag_link' : Since there is a many-to-one relationship between messages and tags I have a third link table. This has two columns, both foreign keys to the messages ('fk_idmessage') and tags tables' ids ('fk_tag').

Inserting messages into the database works fine. I use these following lines in Java:

pst1 = connection.prepareStatement("INSERT INTO messages(message) VALUES (?)");
pst2 = connection.prepareStatement("INSERT INTO tags(tag,count) VALUES (?,1) ON     DUPLICATE KEY UPDATE count = count + 1");
pst3 = connection.prepareStatement("INSERT INTO message_tag_link(fk_idmessage,fk_tag) VALUES (? , ?)");

As can be seen 'count' increments in pst2 every time a duplicate tag entry is made. I am trying to work out how to fully delete a message. I have the on delete cascade description on my foreign keys in my link table's columns but am not sure what to do next. For a given 'messageid' I need to:

1. Delete a row in 'messages' table
2. Delete the associated row in the 'message_tag_link' table
3. Decrement the count value in the 'tags' table when a message deletion is successful, or delete this row entirely if count becomes zero.

Is it possible to do all this within MySQL using a Java PreparedStatement ? Any pointers much appreciated. I am a total MySQL novice, so please be kind!

Answer 1

These queries should do what you need. ? in each query should be the message id that is to be deleted

//Update count
UPDATE tags SET `count` = `count` - 1 WHERE tag_id IN (SELECT fk_tag FROM message_tag_link WHERE fk_idMessage = ?);
//Remove link
DELETE FROM message_tag_link WHERE fk_idmessage = ?;
DELETE FROM messages WHERE id = ?;
//Remove unused tags
DELETE FROM tags WHERE `count` = 0;

As you've found out however maintaining a count can be annoying. Better is to use the link itself.

SELECT COUNT(fk_idmessage) AS msgcount FROM message_tag_link WHERE fk_tag = ?;

Also you can add a CASCADE so that when messages are deleted the tag link gets deleted too (see here ). Be careful with this behavior however).

This solution still leaves tags with no messages but you can remove these using the query:

DELETE FROM tags t WHERE NOT EXISTS (SELECT 1 FROM nessage_tag_link WHERE t.tag_id = fk_tag)