How can we check the indexes created by a UNIQUE constraint on a MySQL table?

SHOW INDEX statement is used to check the indexes created by a UNIQUE constraint on a MySQL table.

Syntax

SHOW INDEX from table_name;

Example

Suppose we have the table ‘empl’ which have a UNIQUE constraint on column ‘empno’.

mysql> describe empl;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| empno  | int(11)     | YES  | UNI | NULL    |       |
| F_name | varchar(20) | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
2 rows in set (0.23 sec)

Now as we know that UNIQUE constraint creates indexes which can be checked form the following query −

mysql> Show Index from empl\G
*************************** 1. row ***************************
       Table: empl
   Non_unique: 0
     Key_name: empno
 Seq_in_index: 1
  Column_name: empno
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment:
Index_comment:
1 row in set (0.02 sec)

The ‘key_name’ in the above result set shows the name of the index and ‘column_name’ is the name of the column on which this index has been added.