SQL Constraints

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SQL Constraints



Primary Key Constraint

Step 1: 

(first ensure the field is NOT NULL)

Syntax to assign to the column the not null constraint:

alter table tablename alter column column int not null 

Step 2:

Syntax:

ALTER TABLE tablename ADD CONSTRAINT constraintname PRIMARY KEY (fieldname);

Example:
Alter table Employees add constraint PK_EmpID PRIMARY KEY (EmployeeID)

Foreign key constraint

Example:

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

Syntax:
Alter table tablename add constraint constraintname default value for column
Example: 
Alter table Orders add constraint df_Constraintname default getdate() for orderdate

 

Check Constraint

Syntax:
Alter table Products add constraint ch_price check (price >= 0)
Example:
Alter table Products add constraint ch_price check (price >= 0)
 

To DROP a Constraint

alter table products drop constraint ch_price

 


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