What is MySQL UNIQUE constraint and how do we apply it to fields of a table?

As the name suggests, MySQL UNIQUE constraints maintain the uniqueness of columns in a table and do not allow the insertion of duplicate values. Basically, the UNIQUE constraint creates an index such that all values ​​in the indexed column must be unique. It is worth mentioning here that there can be multiple UNIQUE columns in a MySQL table.

We can apply UNIQUE constraints by mentioning the "UNIQUE" keyword while defining the column. It can be understood with the help of the following example -

mysql> Create table test3(ID INT UNIQUE, Name Varchar(20));
Query OK, 0 rows affected (0.16 sec)

The above query creates a table named "test3" with "UNIQUE" constraint on the "ID" column. We can check using DESCRIBE statement as shown below-

mysql> DESCRIBE test3;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID    | int(11)     | YES  | UNI | NULL    |       |
| Name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.04 sec)

UNIQUE constraints can also be applied to the columns of the table by following query-

mysql> Create table test4(ID INT, Name Varchar(20),UNIQUE(ID));
Query OK, 0 rows affected (0.15 sec)

We can check using DESCRIBE statement as shown below -

mysql> DESCRIBE test4;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID    | int(11)     | YES  | UNI | NULL    |       |
| Name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.04 sec)

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