Often when developing we will want to change the column types on an existing table. We can do this using the ALTER TABLE and TYPE clauses. In this article, we will learn how to change column data types in Postgresql.
The basic syntax of Change Column Type is as follows:
ALTER TABLE [table_name] ALTER COLUMN column_name TYPE data_type;
We will be using docker in this article, but feel free to install your database locally instead. Once you have docker installed, create a new file called
docker-compose.yml and add the following.
version: '3' services: db: image: 'postgres:latest' ports: - 5432:5432 environment: POSTGRES_USER: username POSTGRES_PASSWORD: password POSTGRES_DB: default_database volumes: - psqldata:/var/lib/postgresql phpmyadmin: image: phpmyadmin/phpmyadmin links: - db environment: PMA_HOST: db PMA_PORT: 3306 PMA_ARBITRARY: 1 restart: always ports: - 8081:80 volumes: psqldata:
Now, navigate to
http://localhost:8081/ to access phpMyAdmin. Then log in with the username
username and pass
Click the SQL tab and you are ready to go.
In this article, we will need some data to work with. If you don't understand these commands, don't worry, we will cover them in later articles.
We will be using the sample db provided here: https://dev.mysql.com/doc/sakila/en/. However, we will only enter what we need rather than import the whole db.
Next, let's create an
employees table. This is a slightly simplified version of the sakila database.
CREATE TABLE employees ( emp_no INT NOT NULL, birth_date DATE NOT NULL, first_name VARCHAR(14) NOT NULL, last_name VARCHAR(16) NOT NULL, gender VARCHAR(1), hire_date DATE NOT NULL, PRIMARY KEY (emp_no) );
Now, let's enter a few rows
INSERT INTO employees VALUES (10001,'1953-09-02','Georgi','Facello','M','1986-06-26'), (10002,'1964-06-02','Bezalel','Simmel','F','1985-11-21'), (10003,'1959-12-03','Parto','Bamford','M','1986-08-28'), (10004,'1954-05-01','Chirstian','Koblick','M','1986-12-01'), (10005,'1955-01-21','Kyoichi','Maliniak','M','1989-09-12');
Now that we are set up, let's see two examples of altering our column's data type.
In this first example, we will update the first name column to use 16 characters instead of 14.
alter table employees alter column first_name type varchar(16);
And for one more example, let's change our gender column to be an integer, in case we want to use numerical enums in our code.
alter table employees alter column gender type int;
Here postgres gives us an error
type varchar and not be automatically converted to type int. So we need to add the
using statement and force cast our type.
alter table employees alter column gender type int using gender::integer;