? ALTER TABLE in SQL

The ALTER TABLE statement is used to change the structure of an existing table — such as adding, modifying, renaming, or deleting columns and constraints.

? Basic Syntax

ALTER TABLE table_name-- perform operation (e.g., ADD, DROP, MODIFY, etc.)

? 1. Add a Column

ALTER TABLE employeesADD birth_date DATE;

? 2. Drop a Column

ALTER TABLE employeesDROP COLUMN birth_date;

? 3. Modify a Column

MySQL:

ALTER TABLE employeesMODIFY COLUMN name VARCHAR(150);

PostgreSQL:

ALTER TABLE employeesALTER COLUMN name TYPE VARCHAR(150);

SQL Server:

ALTER TABLE employeesALTER COLUMN name VARCHAR(150);

? 4. Rename a Column

MySQL 8+ / PostgreSQL / SQL Server:

ALTER TABLE employeesRENAME COLUMN old_name TO new_name;

? 5. Rename a Table

ALTER TABLE old_table_nameRENAME TO new_table_name;

? 6. Add a Constraint

ALTER TABLE employeesADD CONSTRAINT unique_email UNIQUE (email);

? 7. Drop a Constraint

MySQL:

ALTER TABLE employeesDROP INDEX unique_email;

PostgreSQL / SQL Server:

ALTER TABLE employeesDROP CONSTRAINT unique_email;

? Summary Table

Let me know your database type (MySQL, PostgreSQL, SQL Server, etc.) if you want exact syntax for your case!