UNIQUE KEY in SQL.txt

// UNIQUE Key
-> The UNIQUE constraint uniquely identifies each record in a database table.
There can be many UNIQUE constraints per table. A Unique key column can contain NULL values.
ex: CREATE TABLE example(s_id int (10) UNIQUE ,name varchar(50), roll_no
int (10) UNIQUE  ,number int (10));

output:desc example;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| s_id    | int         | YES  | UNI | NULL    |       |
| name    | varchar(50) | YES  |     | NULL    |       |
| roll_no | int         | YES  | UNI | NULL    |       |
| number  | int         | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

ex: insert into example (s_id,name,number) VALUES(2,'RAM',9888888);
mysql> select * from example;
+------+------+---------+---------+
| s_id | name | roll_no | number  |
+------+------+---------+---------+
|    1 | RAM  |       1 | 9888888 |
|    2 | RAM  |    NULL | 9888888 |
+------+------+---------+---------+