How do I change the data type for a column in MySQL?

http://dev.mysql.com/doc/refman/5.1/en/alter-table.html

ALTER TABLE tablename MODIFY columnname INTEGER;

This will change the datatype of given column

Depending on how many columns you wish to modify it might be best to generate a script, or use some kind of mysql client GUI

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alter table table_name modify column_name int(5)

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You can also use this:

ALTER TABLE [tablename] CHANGE [columnName] [columnName] DECIMAL (10,2)

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If you want to change all columns of a certain type to another type, you can generate queries using a query like this:

select distinct concat('alter table ',
                       table_name,
                       ' modify ',
                       column_name,
                       ' <new datatype> ',
                       if(is_nullable = 'NO', ' NOT ', ''),
                       ' NULL;')
  from information_schema.columns
  where table_schema = '<your database>' 
    and column_type = '<old datatype>';

For instance, if you want to change columns from tinyint(4) to bit(1), run it like this:

select distinct concat('alter table ',
                       table_name,
                       ' modify ',
                       column_name,
                       ' bit(1) ',
                       if(is_nullable = 'NO', ' NOT ', ''),
                       ' NULL;')
  from information_schema.columns
  where table_schema = 'MyDatabase' 
    and column_type = 'tinyint(4)';

and get an output like this:

alter table table1 modify finished bit(1)  NOT  NULL;
alter table table2 modify canItBeTrue bit(1)  NOT  NULL;
alter table table3 modify canBeNull bit(1)  NULL;

!! Does not keep unique constraints, but should be easily fixed with another if-parameter to concat. I'll leave it up to the reader to implement that if needed..

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Alter TABLE `tableName` MODIFY COLUMN `ColumnName` datatype(length);

Ex :

Alter TABLE `tbl_users` MODIFY COLUMN `dup` VARCHAR(120);

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To change column data type there are change method and modify method

ALTER TABLE student_info CHANGE roll_no roll_no VARCHAR(255);

ALTER TABLE student_info MODIFY roll_no VARCHAR(255);

To change the field name also use the change method

ALTER TABLE student_info CHANGE roll_no identity_no VARCHAR(255);

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You use the alter table ... change ... method, for example:

mysql> create table yar (id int);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into yar values(5);
Query OK, 1 row affected (0.01 sec)

mysql> alter table yar change id id varchar(255);
Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> desc yar;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | varchar(255) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
1 row in set (0.00 sec)

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https://dev.mysql.com/doc/refman/8.0/en/alter-table.html

You can also set a default value for the column just add the DEFAULT keyword followed by the value.

ALTER TABLE [table_name] MODIFY [column_name] [NEW DATA TYPE] DEFAULT [VALUE];

This is also working for MariaDB (tested version 10.2)

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If you want to alter the column details, set default value and add a comment, use this

ALTER TABLE [table_name] MODIFY [column_name] [new data type] 
DEFAULT [VALUE] COMMENT '[column comment]' 

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