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mysqlsqlkey

SQL keys, MUL vs PRI vs UNI

What is the difference between MUL, PRI and UNI in MySQL?

I'm working on a MySQL query, using the command:

desc mytable;

One of the fields is shown as being a MUL key, others show up as UNI or PRI.

I know that if a key is PRI, only one record per table can be associated with that key. If a key is MUL, does that mean that there could be more than one associated record?

Here's the response of mytable.

+-----------+---------+------+-----+---------+-------+
| Field     | Type    | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| courseid  | int(11) | YES  | MUL | NULL    |       | 
| dept      | char(3) | YES  |     | NULL    |       | 
| coursenum | char(4) | YES  |     | NULL    |       | 
+-----------+---------+------+-----+---------+-------+
Solution

  • It means that the field is (part of) a non-unique index. You can issue 

    show create table <table>;

    To see more information about the table structure.