How do you add or drop a column from a table?

Use ALTER TABLE. Add column: ALTER TABLE users ADD COLUMN phone TEXT;. Add with default and constraint: ALTER TABLE users ADD COLUMN is_active BOOLEAN NOT NULL DEFAULT true;. In PostgreSQL 11+, adding a column with a constant DEFAULT is instant (no table rewrite). Drop column: ALTER TABLE users DROP COLUMN phone;. Drop and cascade to dependent objects: ALTER TABLE users DROP COLUMN phone CASCADE;. Rename column: ALTER TABLE users RENAME COLUMN old_name TO new_name;. Change type: ALTER TABLE users ALTER COLUMN age TYPE BIGINT; — may require a USING clause for non-trivial conversions. Dropping columns doesn't immediately reclaim disk space — run VACUUM FULL afterward if needed.