MySQL unique key only where another key contains a specific value

Question

I have a table which contains meta data for users. There are 4 fields...

`ID`,`meta_name`,`meta_value`,`user_id`

I want to store emails in this table. Naturally these must be unique. However I want to store other data in this table as well, where the data does not need to be unique. Is there any way I can restrict 'meta_value' to be unique, only if 'meta_name' is equal to 'email'?

Answer 1

By means of MySQL's contraints - no.

However you can use insert/update trigger that verifies uniqueness of the data and forbids illegal operations.

Here is a draft of insert trigger (You can play with it here: http://sqlfiddle.com/#!2/5c9e3):

DELIMITER //

CREATE TRIGGER VerifyInsert BEFORE INSERT ON YourTestTable
FOR EACH ROW
BEGIN

IF (SELECT COUNT(1) FROM YourTestTable
WHERE YourTestTable.meta_name = 'EMail' AND YourTestTable.meta_value = NEW.meta_value
AND NEW.meta_name = YourTestTable.meta_name) > 0 THEN

  SIGNAL SQLSTATE '45001' SET MESSAGE_TEXT = 'VerifyInsertFailed';

END IF;


END;

//

Answer 2

With MySQL contraints it's impossible, without special tricks (like redundant column unique_helper with constant value for meta_name=email and random for each other + UNIQUE index for unique_helper+meta_value).

But you can use custom TRIGGER in innoDB tables: (rememeber to put yourTable name)

CREATE TRIGGER triggerName
BEFORE INSERT ON yourTable
FOR EACH ROW
BEGIN
  IF NEW.meta_name = 'email' AND EXISTS (SELECT 1 FROM yourTable WHERE meta_name='email' AND meta_value=NEW.meta_value)
  THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicate email';
  END IF;
END;