Working with Alter Table in Postgres

Intro

The Alter Table allows us to make changes to our SQL tables after they were designed. This is common in applications that are continuously evolving and being built on. With alter table we can change, add, and rename columns. In this article, we will learn how to use Alter Table in PostgreSQL.

The Syntax

The basic syntax of Alter Table is as follows:

ALTER TABLE [table_name]
[MODIFIER]
  modifications
  ...

Getting Setup

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:

Next, run docker-compose up.

Now, navigate to http://localhost:8081/ to access phpMyAdmin. Then log in with the username root and pass root_pass.

Click the SQL tab and you are ready to go.

Creating a DB

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 film 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)
);

CREATE TABLE salaries (
    emp_no      INT             NOT NULL,
    salary      INT             NOT NULL,
    from_date   DATE            NOT NULL,
    to_date     DATE            NOT NULL,
    FOREIGN KEY (emp_no) REFERENCES employees (emp_no) ON DELETE CASCADE,
    PRIMARY KEY (emp_no, from_date)
);

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'),
(10006,'1953-04-20','Anneke','Preusig','F','1989-06-02'),
(10007,'1957-05-23','Tzvetan','Zielinski','F','1989-02-10'),
(10008,'1958-02-19','Saniya','Kalloufi','M','1994-09-15'),
(10009,'1952-04-19','Sumant','Peac','F','1985-02-18'),
(10010,'1963-06-01','Duangkaew','Piveteau','F','1989-08-24'),
(10011,'1953-11-07','Mary','Sluis','F','1990-01-22'),
(10012,'1960-10-04','Patricio','Bridgland','M','1992-12-18'),
(10013,'1963-06-07','Eberhardt','Terkki','M','1985-10-20'),
(10014,'1956-02-12','Berni','Genin','M','1987-03-11'),
(10015,'1959-08-19','Guoxiang','Nooteboom','M','1987-07-02'),
(10016,'1961-05-02','Kazuhito','Cappelletti','M','1995-01-27'),
(10017,'1958-07-06','Cristinel','Bouloucos','F','1993-08-03'),
(10018,'1954-06-19','Kazuhide','Peha','F','1987-04-03'),
(10019,'1953-01-23','Lillian','Haddadi','M','1999-04-30'),
(10020,'1952-12-24','Mayuko','Warwick','M','1991-01-26');

INSERT INTO salaries VALUES (10001,60117,'1986-06-26','1987-06-26'),
(10002,62102,'1987-06-26','1988-06-25'),
(10003,66074,'1988-06-25','1989-06-25'),
(10004,66596,'1989-06-25','1990-06-25'),
(10005,66961,'1990-06-25','1991-06-25'),
(10006,71046,'1991-06-25','1992-06-24'),
(10007,74333,'1992-06-24','1993-06-24'),
(10008,75286,'1993-06-24','1994-06-24'),
(10009,75994,'1994-06-24','1995-06-24'),
(10010,76884,'1995-06-24','1996-06-23'),
(10011,80013,'1996-06-23','1997-06-23'),
(10012,81025,'1997-06-23','1998-06-23'),
(10013,81097,'1998-06-23','1999-06-23');

Adding a Column

There are many modifications we can make to tables. Let's cover a few common modifications in the next few examples.

To start, we can add a new column using the following. This will add a new column for middle names to our employee table.

ALTER TABLE employees 
  ADD COLUMN middle_name varchar(14) NULL;

This will add the column to the end of all the columns by default.

We are not limited to one column. We can add multiple columns by adding another ADD statement.

ALTER TABLE employees 
	ADD COLUMN middle_name varchar(14) NULL,
	ADD COLUMN suffix varchar(14) NULL;

We can few our table details so far by using DESCRIBE.

DESCRIBE employees;

Field	Type	Null	Key
emp_no	int	NO	PRI
birth_date	date	NO
first_name	varchar(14)	NO
suffix	varchar(14)	YES
middle_name	varchar(14)	YES
last_name	varchar(16)	NO
gender	enum('M','F')	NO
hire_date	date	NO

Changing a Column

Let's say we set the number of characters for our middle_name too small. And, for some reason, we want to use numbers for our suffix column. We can use the ALTER COLUMN and TYPE statement to update our column definitions.

To modify one column we can do.

ALTER TABLE employees 
	ALTER COLUMN middle_name TYPE varchar(20);

In the example, we now allow for middle names up to 20 characters. Now, let's change our suffix to an int.

ALTER TABLE employees 
	ALTER COLUMN middle_name TYPE int;

We can change the nullability by using the SET statement.

ALTER TABLE employees 
	ALTER COLUMN middle_name SET not null;

Another example similar to modify is using the renaming columns using the RENAME COLUMN statement.

ALTER TABLE employees 
	RENAME COLUMN middle_name TO middle;

This will change our column middle_name to middle.

Drop a Column

If we want to remove the columns we added, we can do so using the DROP statement.

ALTER TABLE employees 
	DROP COLUMN middle;

Rename a Table

One final example is renaming the table name itself. We can do this using the RENAME TO statement.

ALTER TABLE employees RENAME TO employ;