ALTER TABLE

================

The ALTER TABLE statement is used to modify the structure of an existing table in a database. It is part of the Data Definition Language (DDL) of SQL.

Syntax:

sql
ALTER TABLE table_name
  { ADD | MODIFY | DROP } column_name data_type;

or

sql
ALTER TABLE table_name
  RENAME TO new_table_name;
ALTER TABLE table_name
  CHANGE old_column_name new_column_name data_type;
ALTER TABLE table_name
  ADD CONSTRAINT constraint_name constraint_definition;
ALTER TABLE table_name
  DROP CONSTRAINT constraint_name;

Examples:

### 1. Adding a new column

Suppose we have a table called employees and we want to add a new column called salary.

sql
CREATE TABLE employees (
  id INT PRIMARY KEY,
  name VARCHAR(255),
  department VARCHAR(255)
);
ALTER TABLE employees
ADD salary DECIMAL(10,2);

### 2. Modifying an existing column

Let's say we want to change the data type of the salary column from DECIMAL(10,2) to BIGINT.

sql
ALTER TABLE employees
MODIFY salary BIGINT;

### 3. Dropping a column

Suppose we want to remove the department column from the employees table.

sql
ALTER TABLE employees
DROP COLUMN department;

### 4. Renaming a table

Let's say we want to rename the employees table to staff.

sql
ALTER TABLE employees
RENAME TO staff;

### 5. Adding a primary key constraint

Suppose we want to add a primary key constraint called pk_employees on the id column.

sql
ALTER TABLE employees
ADD CONSTRAINT pk_employees PRIMARY KEY (id);

### 6. Dropping a primary key constraint

Let's say we want to remove the primary key constraint pk_employees.

sql
ALTER TABLE employees
DROP CONSTRAINT pk_employees;

Note that these examples demonstrate basic usage of the ALTER TABLE statement. The actual syntax and behavior may vary depending on the specific database management system (DBMS) being used.