How to set primary key constraints in mysql

Instructions

1. Use the UNIQUE keyword to specify unique constraints directly after defining the column.

The difference between UNIQUE and PRIMARY KEY: A table can have multiple fields declared as UNIQUE, but there can only be one PRIMARY KEY declaration.

2. Columns declared as PRIMAY KEY do not allow null values, but fields declared as UNIQUE allow null values.

Example

mysql> CREATE TABLE demo_department
    -> (
    -> id INT(11) PRIMARY KEY,
    -> name VARCHAR(22) UNIQUE,
    -> location VARCHAR(50)
    -> );
Query OK, 0 rows affected (0.37 sec)
mysql> DESC demo_department;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | NO   | PRI | NULL    |       |
| name     | varchar(40) | YES  | UNI | NULL    |       |
| location | varchar(50) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.08 sec)

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