SQL keys, MUL vs PRI vs UNI

Question

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

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

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Answer 1

UNI: For UNIQUE:

• It is a set of one or more columns of a table to uniquely identify the record.
• A table can have multiple UNIQUE key.
• It is quite like primary key to allow unique values but can accept one null value which primary key does not.

PRI: For PRIMARY:

• It is also a set of one or more columns of a table to uniquely identify the record.
• A table can have only one PRIMARY key.
• It is quite like UNIQUE key to allow unique values but does not allow any null value.

MUL: For MULTIPLE:

• It is also a set of one or more columns of a table which does not identify the record uniquely.
• A table can have more than one MULTIPLE key.
• It can be created in table on index or foreign key adding, it does not allow null value.
• It allows duplicate entries in column.
• If we do not specify MUL column type then it is quite like a normal column but can allow null entries too hence; to restrict such entries we need to specify it.
• If we add indexes on column or add foreign key then automatically MUL key type added.