Page 22 - SQL

Chapter 2: ALTER TABLE

Introduction

ALTER command in SQL is used to modify column/constraint in a table

Syntax

• ALTER TABLE [table_name] ADD [column_name] [datatype]

Examples

Add Column(s)

ALTER TABLE Employees
ADD StartingDate date NOT NULL DEFAULT GetDate(),
DateOfBirth date NULL

The above statement would add columns named StartingDate which cannot be NULL with default
value as current date and DateOfBirth which can be NULL in Employees table.

Drop Column

ALTER TABLE Employees
DROP COLUMN salary;

This will not only delete information from that column, but will drop the column salary from table
employees(the column will no more exist).

Drop Constraint

ALTER TABLE Employees
DROP CONSTRAINT DefaultSalary

This Drops a constraint called DefaultSalary from the employees table definition.

Note:- Ensure that constraints of the column are dropped before dropping a column.

Add Constraint

ALTER TABLE Employees
ADD CONSTRAINT DefaultSalary DEFAULT ((100)) FOR [Salary]

This adds a constraint called DefaultSalary which specifies a default of 100 for the Salary column.

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