Constraints in TSQL

Below are the List of Adding Constraints in a table

To add referential Integrity between tables
--foreign key constraint
alter table sales.orders
add constraint FK_orderrs
Foreign key(empid)
references dbo.employee(empid)

To Check all the values of a column are unique it can take one NULL Values
--unique constraint
alter table dbo.employee
add constraint UQ_Emp_SN
unique (ssn)

To Set Default value to a column if no value will be inserted or updated it will take default values

--default constraint
alter table orders
add constraint df_dt
default(getdate()) for orders

To Check Value of a column every time with insert and update it will check for the value

--check constaint
ALTER TABLE dbo.Employees 
ADD CONSTRAINT CHK_Employees_salary 
CHECK(salary > 0);


To add Primary key Constraint in a table it gaurntee uniqueness without taking any Null Value
--primary key constraint
ALTER TABLE dbo.Employees 
ADD CONSTRAINT PK_Employees 
PRIMARY KEY(empid);



To add Primary Key Constraint while creating a table
--Primary key constraints while creating table
CREATE TABLE dbo.Orders ( orderid INT  NOT NULL, empid INT  NOT NULL,
custid VARCHAR(10) NOT NULL, orderts  DATETIME NOT NULL, qty INT  NOT NULL, 
CONSTRAINT PK_Orders PRIMARY KEY(OrderID) );