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!