SQL keys, MUL vs PRI vs UNI
This is acutally a shortcut for:
SHOW COLUMNS FROM <table>;
In any case, there are three possible values for the "Key" attribute:
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:http://dev.mysql.com/doc/refman/5.1/en/show-columns.html
"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!