ALTER TABLE COLUMN
Introduced or updated: v1.2.50
Modifies a table by adding, converting, renaming, removing, or changing the type of a column.
Syntax
-- Add a column to the end of the table
ALTER TABLE [IF EXISTS] [database.]<table_name>
ADD COLUMN <column_name> <data_type> [NOT NULL | NULL] [DEFAULT <constant_value>];
-- Add a column to a specified position
ALTER TABLE [IF EXISTS] [database.]<table_name>
ADD COLUMN <column_name> <data_type> [NOT NULL | NULL] [DEFAULT <constant_value>] [FIRST | AFTER <column_name>]
-- Add a virtual computed column
ALTER TABLE [IF EXISTS] [database.]<table_name>
ADD COLUMN <column_name> <data_type> AS (<expr>) VIRTUAL;
-- Convert a stored computed column to a regular column
ALTER TABLE [IF EXISTS] [database.]<table_name>
MODIFY COLUMN <column_name> DROP STORED;
-- Rename a column
ALTER TABLE [IF EXISTS] [database.]<table_name>
RENAME COLUMN <column_name> TO <new_column_name>;
-- Change the data type of one or multiple columns
ALTER TABLE [IF EXISTS] [database.]<table_name>
MODIFY COLUMN <column_name> <new_data_type>[, COLUMN <column_name> <new_data_type>, ...]
-- Remove a column
ALTER TABLE [IF EXISTS] [database.]<table_name>
DROP COLUMN <column_name>;
note
- Only a constant value can be accepted as a default value when adding a new column. If a non-constant expression is used, an error will occur.
- Adding a stored computed column with ALTER TABLE is not supported yet.
- When you change the data type of a table's columns, there's a risk of conversion errors. For example, if you try to convert a column with text (String) to numbers (Float), it might cause problems.
Examples
Example 1: Adding, Renaming, and Removing a Column
This example illustrates the creation of a table called "default.users" with columns 'username', 'email', and 'age'. It showcases the addition of columns 'id' and 'middle_name' with various constraints. The example also demonstrates the renaming and subsequent removal of the "age" column.
-- Create a table
CREATE TABLE default.users (
username VARCHAR(50) NOT NULL,
email VARCHAR(255),
age INT
);
-- Add a column to the end of the table
ALTER TABLE default.users
ADD COLUMN business_email VARCHAR(255) NOT NULL DEFAULT 'example@example.com';
DESC default.users;
Field |Type |Null|Default |Extra|
--------------+-------+----+---------------------+-----+
username |VARCHAR|NO |'' | |
email |VARCHAR|NO |'' | |
age |INT |NO |0 | |
business_email|VARCHAR|NO |'example@example.com'| |
-- Add a column to the beginning of the table
ALTER TABLE default.users
ADD COLUMN id int NOT NULL FIRST;
DESC default.users;
Field |Type |Null|Default |Extra|
--------------+-------+----+---------------------+-----+
id |INT |NO |0 | |
username |VARCHAR|NO |'' | |
email |VARCHAR|NO |'' | |
age |INT |NO |0 | |
business_email|VARCHAR|NO |'example@example.com'| |
-- Add a column after the column 'username'
ALTER TABLE default.users
ADD COLUMN middle_name VARCHAR(50) NULL AFTER username;
DESC default.users;
Field |Type |Null|Default |Extra|
--------------+-------+----+---------------------+-----+
id |INT |NO |0 | |
username |VARCHAR|NO |'' | |
middle_name |VARCHAR|YES |NULL | |
email |VARCHAR|NO |'' | |
age |INT |NO |0 | |
business_email|VARCHAR|NO |'example@example.com'| |
-- Rename a column
ALTER TABLE default.users
RENAME COLUMN age TO new_age;
DESC default.users;
Field |Type |Null|Default |Extra|
--------------+-------+----+---------------------+-----+
id |INT |NO |0 | |
username |VARCHAR|NO |'' | |
middle_name |VARCHAR|YES |NULL | |
email |VARCHAR|NO |'' | |
new_age |INT |NO |0 | |
business_email|VARCHAR|NO |'example@example.com'| |
-- Remove a column
ALTER TABLE default.users
DROP COLUMN new_age;
DESC default.users;
Field |Type |Null|Default |Extra|
--------------+-------+----+---------------------+-----+
id |INT |NO |0 | |
username |VARCHAR|NO |'' | |
middle_name |VARCHAR|YES |NULL | |
email |VARCHAR|NO |'' | |
business_email|VARCHAR|NO |'example@example.com'| |
Example 2: Adding a Computed Column
This example demonstrates creating a table for storing employee information, inserting data into the table, and adding a computed column to calculate the age of each employee based on their birth year.
-- Create a table
CREATE TABLE Employees (
ID INT,
Name VARCHAR(50),
BirthYear INT
);
-- Insert data
INSERT INTO Employees (ID, Name, BirthYear)
VALUES
(1, 'John Doe', 1990),
(2, 'Jane Smith', 1985),
(3, 'Robert Johnson', 1982);
-- Add a computed column named Age
ALTER TABLE Employees
ADD COLUMN Age INT64 AS (2023 - BirthYear) VIRTUAL;
SELECT * FROM Employees;
ID | Name | BirthYear | Age
------------------------------------
1 | John Doe | 1990 | 33
2 | Jane Smith | 1985 | 38
3 | Robert Johnson| 1982 | 41
Example 3: Converting a Computed Column
This example creates a table called "products" with columns for ID, price, quantity, and a computed column "total_price." The ALTER TABLE statement removes the computed functionality from the "total_price" column, converting it into a regular column.
CREATE TABLE IF NOT EXISTS products (
id INT,
price FLOAT64,
quantity INT,
total_price FLOAT64 AS (price * quantity) STORED
);
ALTER TABLE products
MODIFY COLUMN total_price DROP STORED;
Example 4: Changing Data Type of a Column
This example creates a table named "students_info" with columns for "id," "name," and "age," inserts some sample data, and then modifies the data type of the "age" column from INT to VARCHAR(10).
CREATE TABLE students_info (
id INT,
name VARCHAR(50),
age INT
);
INSERT INTO students_info VALUES
(1, 'John Doe', 25),
(2, 'Jane Smith', 28),
(3, 'Michael Johnson', 22);
ALTER TABLE students_info MODIFY COLUMN age VARCHAR(10);
SELECT * FROM students_info;
id|name |age|
--+---------------+---+
1|John Doe |25 |
2|Jane Smith |28 |
3|Michael Johnson|22 |