如果存在则更新一行，否则插入

Question

I have a table 'match' like

id|user1|user2|paired
--+-----+-----+--------+
1 |U_1  |null |false

I need to match a new user 'U_2' to a record where paired = false, or create a new entry in table if no unpaired row is found.

This db is connected to a server where multiple users might be trying to get paired, so I need to find best possible solution that makes it fast so it doesn't lock the table for long.

the solution I came up with was

int matchId = select id from match where ((user1 != 'U_2') AND (paired = false));

if(matchId > 0)
then
   update table match set user2 = 'U_2' where id = matchId; 
else
   insert new row.

Please suggest a better way.

Thanks in advance.

Answer 1

You can

• add unique indices for user1 and user2 to improve speed and assure integrity.
• use Transaction to avoid collisions.

• combine the select and update query in one update:

   update table match set user2 = 'U_2' where ((user1 != 'U_2') AND (paired = false)) LIMIT 1; 

• check if the update has affected rows . If not, insert the new row.

If i understand your intension properly, you can also:

• remove the column paired , it seems to be redundant, since it is always false when user2=null

Answer 2

A single statement does one or the other:

INSERT INTO match
    (user1, paired, user2)
    VALUES
    ('U_2', false, 'U_2')   -- either insert this
ON DUPLICATE KEY UPDATE
    user2 = VALUES(user2);  -- or update this

Together with

PRIMARY KEY(user1, paired)  -- a UNIQUE key to control what is "DUPLICATE"