Rename column

Column and Table Aliases

syntax

SELECT column AS alias_name FROM table AS t;

example

SELECT
  u.first_name AS name,
  o.total_amount AS order_total
FROM users AS u
JOIN orders AS o ON o.user_id = u.id;

output

-- name  | order_total
-- Alice | 149.99

Note AS is optional in most databases but improves clarity. You cannot reference a column alias in the WHERE clause of the same query — use a subquery or CTE instead.

ALTER TABLE

SQL · Table Operations

syntax

ALTER TABLE table_name
  ADD COLUMN col type constraints,
  DROP COLUMN col,
  ALTER COLUMN col SET DATA TYPE new_type,
  ADD CONSTRAINT name constraint_definition;

example

ALTER TABLE users
  ADD COLUMN phone VARCHAR(20),
  ADD COLUMN updated_at TIMESTAMP;

ALTER TABLE users
  ALTER COLUMN email SET NOT NULL;

ALTER TABLE users
  ADD CONSTRAINT chk_email CHECK (email LIKE '%@%');

output

-- Adds columns, modifies existing, adds constraint

Note ALTER TABLE syntax varies across databases. PostgreSQL uses ALTER COLUMN ... TYPE. MySQL uses MODIFY COLUMN. Adding NOT NULL to a column with existing NULLs will fail — update the data first.

SQLSQL