Question

I have run into a problem where the easiest solution would be to add a unique index that goes across multiple tables in mysql. Is this possible?

For the purpose of this question I will have three tables; status, tracks and events. The status and tracks tables both have an auto incrementing ID (T_ID or S_ID) in. Information from them is added to the events table with a trigger. The problem is that there could be the same auto incrementing ID in tracks and status, this means that their could be the same ID more than once in events.

tracks;

CREATE TABLE `tracks` (
`ID` int(11) NOT NULL,
`url` varchar(200) COLLATE latin1_general_ci NOT NULL,
`name` varchar(100) COLLATE latin1_general_ci NOT NULL,
`timestamp` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' ON UPDATE CURRENT_TIMESTAMP,
`T_ID` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`T_ID`),
UNIQUE KEY `url` (`url`),
UNIQUE KEY `T_ID` (`T_ID`),
KEY `ID` (`ID`),
CONSTRAINT `tracks_ibfk_1` FOREIGN KEY (`ID`) REFERENCES `members` (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci

status

CREATE TABLE `status` (
`ID` int(11) NOT NULL,
`status` varchar(300) COLLATE latin1_general_ci NOT NULL,
`timestamp` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' ON UPDATE CURRENT_TIMESTAMP,
`S_ID` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`S_ID`),
UNIQUE KEY `S_ID` (`S_ID`),
KEY `ID` (`ID`),
CONSTRAINT `status_ibfk_1` FOREIGN KEY (`ID`) REFERENCES `artists` (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci

events

CREATE TABLE `events` (
`ID` int(11) NOT NULL,
`action` varchar(100) COLLATE latin1_general_ci NOT NULL,
`timestamp` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' ON UPDATE CURRENT_TIMESTAMP,
`E_ID` int(11) NOT NULL,
KEY `ID` (`ID`),
CONSTRAINT `events_ibfk_1` FOREIGN KEY (`ID`) REFERENCES `members` (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci COMMENT='This table shows what the artist has done and is used feed'

Answer 1

Agreed, you cannot have an index across multiple tables.

However, consider changing your schema by adding a “source” column to your event table and making a unique constraint on the combination of the source and the foreign key. This would prevent duplicate keys being inserted from the same source.