PostgreSQL – How to add Column to a Table

To add a new column to an existing table in PostgreSQL, you can use the ALTER TABLE statement.

Below is the basic syntax of adding a new column:

ALTER TABLE table_name ADD COLUMN new_column_name data_type;

where table_name is the name of the table to which you want to add the new column.

new_column_name is the name you want to give to the new column.

data_type is the data type of the new column (e.g., INTEGER, VARCHAR(255), DATE, etc.).

For example, if you have a table named employees and you want to add a new column named salary with a data type of INTEGER, you would run the following SQL command:

ALTER TABLE employees ADD COLUMN salary INTEGER;

If the new column has a default value, you can provide it in the ADD COLUMN statement:

ALTER TABLE employees ADD COLUMN salary INTEGER DEFAULT 0;

Adding a column might lock the table briefly while the alteration is performed, which could affect concurrent access to the table. Always make sure to have backups before making significant changes to your database schema.

Below is the syntax to add multiple columns to a table:

ALTER TABLE employees 
   ADD COLUMN salary INTEGER DEFAULT 0,
   ADD COLUMN age INTEGER ,
   ...
   ADD COLUMN department varchar(20);  

Data Types: When adding a new column, you need to specify the data type for that column. PostgreSQL supports various data types such as INTEGER, VARCHAR, TEXT, DATE, TIMESTAMP, and many others. Choose a data type that best fits the kind of data you’ll be storing in the new column.

Position of the New Column: By default, the new column will be added at the end of the table’s column list. If you want to specify a specific position for the new column, you can use the AFTER keyword followed by the column name after which you want the new column to be placed. For example:

ALTER TABLE employees ADD COLUMN salary INTEGER AFTER last_name;

Default Values: You can specify a default value for the new column using the DEFAULT keyword. This default value will be used for existing rows that don’t have a value for the new column. For example:

ALTER TABLE employees ADD COLUMN salary INTEGER DEFAULT 0;

Adding Constraints: You can also add constraints to the new column, such as NOT NULL or UNIQUE, when adding it. However, adding a NOT NULL column without a default value can be tricky, as it requires you to either populate existing rows with values or provide a default that satisfies the constraint.

ALTER TABLE employees ADD COLUMN salary INTEGER DEFAULT 0 NOT NULL;

Concurrent Considerations: Adding a column to a large table could take some time, during which the table might be locked for write operations. If your table is frequently accessed, consider using tools like pg_repack or pg_reorg to minimize downtime during such changes.

Indexes: If you add a column that you plan to frequently use in search conditions (e.g., in WHERE clauses), consider creating an index on that column to improve query performance.

Backups and Testing: Before making any changes to your database schema, especially when altering tables and adding columns, it’s a good practice to create backups of your database. Additionally, it’s a good idea to test these changes in a development or staging environment before applying them to production.

Schema changes have the potential to impact application behavior and performance, so it’s important to plan and execute them carefully.