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How add unique key to existing table (with non-unique rows)?
You can add unique key to existing table with the help of alter command. The syntax is as follows −
ALTER TABLE yourTableName ADD CONSTRAINT yourConstraintName UNIQUE(yourColumnName1,yourColumnName2,............N);
To understand the above concept, let us create a table with some columns. The query to create a table −
mysql> create table MovieCollectionDemo −> ( −> MovieId int, −> MovieDirectorName varchar(200), −> NumberOfSongs int unsigned −> ); Query OK, 0 rows affected (0.62 sec)
Now you can check the table does not have any unique constraint. The query to check unique constraint is as follows −
mysql> desc MovieCollectionDemo;
The following is the output −
+-------------------+------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------------+------------------+------+-----+---------+-------+ | MovieId | int(11) | YES | | NULL | | | MovieDirectorName | varchar(200) | YES | | NULL | | | NumberOfSongs | int(10) unsigned | YES | | NULL | | +-------------------+------------------+------+-----+---------+-------+ 3 rows in set (0.00 sec)
Now you can add unique key from the above syntax. We are adding unique key on column MovieId. The query is as follows −
mysql> alter table MovieCollectionDemo add constraint uni_moviecollectio unique(MovieId); Query OK, 0 rows affected (0.46 sec) Records: 0 Duplicates: 0 Warnings: 0
Let us look at the entire table and the column MovieId to check whether it has unique key or not.
mysql> desc MovieCollectionDemo;
The following is the output −
+-------------------+------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------------+------------------+------+-----+---------+-------+ | MovieId | int(11) | YES | UNI | NULL | | | MovieDirectorName | varchar(200) | YES | | NULL | | | NumberOfSongs | int(10) unsigned | YES | | NULL | | +-------------------+------------------+------+-----+---------+-------+ 3 rows in set (0.02 sec)
The above output displays the “UNI”, that means the field “MovieId” has Unique Key.
Updated on: 25-Jun-2020
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