PostgreSQL: Changing Column Data Types with ALTER TABLE

Modify column data types in PostgreSQL tables using `ALTER TABLE ALTER COLUMN TYPE`. This guide explains the process and potential considerations.

Changing Column Data Types in PostgreSQL

In PostgreSQL, you can modify a table's column data type using the `ALTER TABLE ALTER COLUMN TYPE` command. This is a crucial database operation for adapting your database schema to changing requirements. However, be mindful of potential data loss or errors when changing column types.

`ALTER TABLE ALTER COLUMN TYPE` Syntax


ALTER TABLE table_name ALTER COLUMN column_name TYPE new_data_type;

This command changes the data type of `column_name` in `table_name` to `new_data_type`. You can use `SET DATA TYPE` instead of `TYPE`.

Modifying Multiple Columns

To modify multiple columns in a single command, use multiple `ALTER COLUMN` clauses separated by commas:

ALTER TABLE table_name
ALTER COLUMN column1 TYPE new_type1,
ALTER COLUMN column2 TYPE new_type2;

    

Using the `USING` Clause

PostgreSQL might attempt to implicitly convert existing data to the new type. To explicitly define the conversion, use the `USING` clause:

ALTER TABLE table_name ALTER COLUMN column_name TYPE new_data_type USING expression;

    

If the implicit conversion fails, you'll get an error, and the `USING` clause becomes mandatory.

Example: Altering Column Data Types

(Note: This example involves creating a table and inserting data. Screenshots from the original text are not included here. Please refer to the original document for visual confirmation of the commands' success. The descriptions below aim to convey the information in those screenshots.)

Create Table: Create a `Student_information` table.
Insert Data: Add some sample student data.
Change Single Column Type: Change the `Stu_name` column's type to `VARCHAR`.
Change Multiple Column Types: Change the `Stu_age` and `Stu_address` column types.

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