ALTER COMMAND is used to modify an existing MySQL column's data type. Following is an example which demonstrates that how we can use this command to modify column's data type −
mysql> describe testing\G *************************** 1. row *************************** Field: id1 Type: int(11) Null: NO Key: PRI Default: 0 Extra: *************************** 2. row *************************** Field: name Type: char(30) Null: YES Key: Default: NULL Extra: 2 rows in set (0.05 sec)
From the above DESCRIBE query, we can see that the data type of the Name column is CHAR(30). Now with the help of the following query we can change it to VARCHAR(20) −
mysql> ALTER TABLE Testing MODIFY Name Varchar(20); Query OK, 4 rows affected (0.60 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql> Describe Testing\G; *************************** 1. row *************************** Field: id1 Type: int(11) Null: NO Key: PRI Default: 0 Extra: *************************** 2. row *************************** Field: Name Type: varchar(20) Null: YES Key: Default: NULL Extra: 2 rows in set (0.15 sec)
Now the data type has been modified to VARCHAR(20).
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