How can we create multicolumn UNIQUE indexes?
For creating multicolumn UNIQUE indexes we need to specify an index name on more than one column. Following example will create a multicolumn index named ‘id_fname_lname’ on the columns ‘empid’, ’first_name’, ’last_name’ of ‘employee’ table −
mysql> Create UNIQUE INDEX id_fname_lname on employee(empid,first_name,last_name);
Query OK, 0 rows affected (0.41 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> describe employee;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| empid | int(11) | YES | MUL | NULL | |
| first_name | varchar(20) | YES | | NULL | |
| last_name | varchar(20) | YES | | NULL | |
+------------+-------------+------+-----+---------+-------+
3 rows in set (0.12 sec)
From the result set of above query, we can see that a multiple index is defined on the table. Forgetting the details about the indexes we can run the following query −
mysql> Show index from employee\G
*************************** 1. row ***************************
Table: employee
Non_unique: 0
Key_name: id_fname_lname
Seq_in_index: 1
Column_name: empid
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 2. row ***************************
Table: employee
Non_unique: 0
Key_name: id_fname_lname
Seq_in_index: 2
Column_name: first_name
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
*************************** 3. row ***************************
Table: employee
Non_unique: 0
Key_name: id_fname_lname
Seq_in_index: 3
Column_name: last_name
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
3 rows in set (0.00 sec)
We can observe from the above result set that the value in the ‘key_name’ filed is same because we have created the multicolumn index on all the columns of the table.
Published on 16-Feb-2018 17:54:15
