SQL keys MUL vs PRI vs UNI

Question

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

I'm working with the command: while running a MySQL query.

desc mytable; 

One of the fields appears to be a MUL key, while the others display as UNI or PRI.

I am aware that only one record per table can be linked to a key if it is PRI. Is it possible for there to be more than one associated record if a key is MUL?

This is mytable's reaction.

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

Can someone please help me with this?

Answer 1

It denotes that the field is (a part of) an index that isn't unique. You can issue

show create table <table>;

To see more information about the table structure. I hope this helps you.