SQL Constraints
SQL Constraints Primary Key Constraint Step 1: (first ensure the field is NOT NULL) alter table tablename alter column column int not null Step 2: ALTER TABLE tablename ADD CONSTRAINT constraintname PRIMARY KEY (fieldname ); Example: Alter table Employees add constraint PK_EmpID PRIMARY KEY ( EmployeeID ) Foreign key constraint alter table Orders -- table name add constraint FK_EID – constraint name foreign key (ShipperID) -- column that will become a foreign key references Shippers (ShipperID) -– table and column name to which it links Default constraint Alter table Orders add constraint df_Constraintname default getdate() for orderdate Check Constraint Alter table Products add constraint ch_price check (price >= 0) To DROP a Constraint alter table products drop constraint ch_price