What does "MUL" mean in MySQL for the key?

In any case, there are three possible values for the “Key” attribute:

  1. PRI
  2. UNI
  3. MUL

The meaning of PRI and UNI are quite clear:

  • PRI=> primary key
  • UNI=> unique key

The third possibility, MUL, (which you asked about) is basically an index that is neither a primary key nor a unique key. The name comes from “multiple” because multiple occurences of the same value are allowed. Straight from the MySQL documentation:

“If Key is MUL, the column is the first column of a nonunique index in which multiple occurrences of a given value are permitted within the column.”

There is also a final caveat:

“If more than one of the Key values applies to a given column of a table, Key displays the one with the highest priority, in the order PRI, UNI, MUL.”

As a general note, the MySQL documentation is quite good. When in doubt, check it out!

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3 thoughts on “What does "MUL" mean in MySQL for the key?

  1. MUL is a Foreign Key of some Primary key

    its My Table

    CREATE TABLE `employees` (
    `employeeNumber` int(11) NOT NULL,
    `lastName` varchar(50) NOT NULL,
    `firstName` varchar(50) NOT NULL,
    `extension` varchar(10) NOT NULL,
    `email` varchar(100) NOT NULL,
    `officeCode` varchar(10) NOT NULL,
    `reportsTo` int(11) DEFAULT NULL,
    PRIMARY KEY (`employeeNumber`),
    KEY `reportsTo` (`reportsTo`),
    KEY `officeCode` (`officeCode`),
    CONSTRAINT `employees_ibfk_1` FOREIGN KEY (`reportsTo`) REFERENCES `employees` (`employeeNumber`),
    CONSTRAINT `employees_ibfk_2` FOREIGN KEY (`officeCode`) REFERENCES `offices` (`officeCode`)
    ) ENGINE=InnoDB DEFAULT CHARSET=latin1

    And its a Desc

    “Field”: “officeCode”,
    “Type”: “varchar(10)”,
    “Null”: “NO”,
    “Key”: “MUL”,
    “Default”: null,
    “Extra”: “”

    MUL Allows you to Enter multiple same Value Which is present in Primary Key

    Like

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