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;

//