Nisarg Upadhyay
SQL Query to rename the column

SQL queries to change the column type

September 22, 2021 by

This article covers the different SQL queries to change the column type. We are going to learn how we can change the data type of the columns of the following databases:

  1. SQL Server 2019
  2. MySQL Server
  3. PostgreSQL

SQL query to change the column type in SQL Server database

We can use ALTER TABLE ALTER COLUMN statement to change the column type of the table. The syntax to change the column type is following:

In the syntax,

  • Tbl_name: Specify the table name
  • Col_name: Specify the column name whose datatype you want to change. The col_name must be specified after the ALTER COLUMN keyword
  • Datatype: Specify the new datatype and length of the column

For demonstration, I have created a table named tblStudent.

Suppose you want to change the data type of [address] from varchar(max) to nvarchar(1500). Run the following query to change the column type.

Verify the changes by running following script.

SQL Query to change column type in SQL Server

As you can see, the column data type has been changed.

Important Notes:

  1. When we decrease the size of the column, the SQL Server will check the data of the table, and if the length of the data is higher than the new length, it returns the warning and terminate the statement
  2. When you change the datatype from nvarchar to varchar, and the column contains the Unicode string, then SQL Server returns the error and terminates the statement
  3. Unlike MySQL, the changing datatype of multiple columns is not allowed
  4. You cannot add
    1. NOT NULL constraint if the column contains the NULL values
    2. UNIQUE constraint if the column has duplicate values

SQL query to change the column type in MySQL Server

We can use ALTER TABLE MODIFY COLUMN statement to change the datatype of the column. The syntax to change the datatype of the column is following.

In the syntax,

  • Tbl_name: Specify the table name that contains the column that you want to change
  • Col_name: Specify the column name whose datatype you want to change. The col_name must be specified after the MODIFY COLUMN keyword. We can change the data type of multiple columns. When we change the datatype of multiple columns, each column must be separated with a comma (,)
  • Datatype: Specify the new datatype and length of the column. The datatype must be specified after the column name

For demonstration, I have created a table named tblactor in DemoDatabase. The code to create the table is the following

Now, let us understand the concept using a few examples.

Example 1: SQL query to change the datatype of one column

We want to change the column type of the address column from varchar(500) to TEXT datatype. Run the following query to change the datatype.

Run the following query to verify the changes:

SQL Query to change column type of single column

As you can see, the datatype of the address column has been changed to TEXT.

Example 2: SQL query to change the datatype of multiple columns

We can change the datatype of multiple columns of a table. In our example, we want to change the column type of the first_name and last_name. The new datatype of the columns is varchar(200).

Run the following query to verify the changes:

SQL Query to change the column type of multiple columns

As you can see, the datatype of the first_name and last_name column has been changed to TINYTEXT.

Example 3: Rename the column in MySQL Server

To rename the columns, we must use ALTER TABLE CHANGE COLUMN statement. Suppose you want to rename the column CityID to CityCode, you must execute the following query.

Run the describe command to view the changes in table structure.

SQL Query to rename the column

As you can see, the column name has been changed.

SQL query to change the column type in PostgreSQL database

We can use ALTER TABLE ALTER COLUMN statement to change the datatype of the column. The syntax to change the datatype of the column is the following.

In the syntax,

  • Tbl_name: Specify the table name that contains the column that you want to change
  • Col_name: Specify the column name whose datatype you want to change. The col_name must be specified after the ALTER COLUMN keyword. We can change the data type of multiple columns
  • Datatype: Specify the new datatype and length of the column. The datatype must be specified after the TYPE keyword

For demonstration, I have created a table named tblmovies in DemoDatabase. The code to create the table is the following.

Now, let us understand the concept using a few examples.

Example 1: SQL query to change the datatype of one column

We want to change the column type of the movie_id column from int4 to int8 data type. Run the following query to change the datatype.

Run the following query to verify the changes:

SQL Query to change column type of single column in Postgresql

As you can see, the datatype of the movie_id column has been changed to int8.

Example 2: SQL query to change the datatype of multiple columns

We can change the datatype of multiple columns of a table. In our example, we want to change the column type of the movie_title and movie_director. The new datatype of the movie_title columns is TEXT, and the new datatype of movie_producer is varchar(2000).

Run the following query to verify the changes:

SQL Query to change column type of multiple column in Postgresql

As you can see, the datatype of the movie_title columns is TEXT, and the datatype of movie_producer is varchar(2000).

Summary

In this article, we learned how we could change the data type of the columns in SQL Server 2019, MySQL Server, and PostgreSQL.

Nisarg Upadhyay
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