SQL Constraints

Constraints are used to apply business rules for the database tables.
The constraints available in SQL are Foreign Key, Not Null, Unique, Check.

1) Primary key

This constraint defines a column or combination of columns which uniquely identifies each row in the table.
Syntax to define a Primary key at column level:

column name datatype [CONSTRAINT constraint_name] PRIMARY KEY

Syntax to define a Primary key at table level:

[CONSTRAINT constraint_name] PRIMARY KEY (column_name1,column_name2,..)

• column_name1, column_name2 are the names of the columns which define the primary Key.
• The syntax within the bracket i.e. [CONSTRAINT constraint_name] is optional.

For Example: To create an employee table with Primary Key constraint, the query would be like.
Primary Key at column level:

CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10)
);


or

CREATE TABLE employee
( id number(5) CONSTRAINT emp_id_pk PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10)
);


Primary Key at column level:

CREATE TABLE employee
( id number(5),
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10),
CONSTRAINT emp_id_pk PRIMARY KEY (id)
);


Primary Key at table level:

CREATE TABLE employee
( id number(5), NOT NULL,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10),
ALTER TABLE employee ADD CONSTRAINT PK_EMPLOYEE_ID PRIMARY KEY (id)
);


2) Foreign key 

This constraint identifies any column referencing the PRIMARY KEY in another table. It establishes a relationship between two columns in the same table or between different tables. For a column to be defined as a Foreign Key, it should be a defined as a Primary Key in the table which it is referring. One or more columns can be defined as Foreign key.
Syntax to define a Foreign key at column level:

[CONSTRAINT constraint_name] REFERENCES Referenced_Table_name(column_name)

Syntax to define a Foreign key at table level:

[CONSTRAINT constraint_name] FOREIGN KEY(column_name) REFERENCES referenced_table_name(column_name);

For Example:
1) Lets use the "product" table and "order_items".
Foreign Key at column level:

CREATE TABLE product
( product_id number(5) CONSTRAINT pd_id_pk PRIMARY KEY,
product_name char(20),
supplier_name char(20),
unit_price number(10)
);


CREATE TABLE order_items
( order_id number(5) CONSTRAINT od_id_pk PRIMARY KEY,
product_id number(5) CONSTRAINT pd_id_fk REFERENCES, product(product_id),
product_name char(20),
supplier_name char(20),
unit_price number(10)
);


Foreign Key at table level:

CREATE TABLE order_items
( order_id number(5) ,
product_id number(5),
product_name char(20),
supplier_name char(20),
unit_price number(10)
CONSTRAINT od_id_pk PRIMARY KEY(order_id),
CONSTRAINT pd_id_fk FOREIGN KEY(product_id) REFERENCES product(product_id)
);


2) If the employee table has a 'mgr_id' i.e, manager id as a foreign key which references primary key 'id' within the same table, the query would be like,

CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
mgr_id number(5) REFERENCES employee(id),
salary number(10),
location char(10)
);


3) Not Null

This constraint ensures all rows in the table contain a definite value for the column which is specified as not null. Which means a null value is not allowed.
Syntax to define a Not Null constraint:

[CONSTRAINT constraint name] NOT NULL

For Example: To create a employee table with Null value, the query would be like

CREATE TABLE employee
( id number(5),
name char(20) CONSTRAINT nm_nn NOT NULL,
dept char(10),
age number(2),
salary number(10),
location char(10)
);


4) Unique key

This constraint ensures that a column or a group of columns in each row have a distinct value. A column(s) can have a null value but the values cannot be duplicated.
Syntax to define a Unique key at column level:

[CONSTRAINT constraint_name] UNIQUE

Syntax to define a Unique key at table level:

[CONSTRAINT constraint_name] UNIQUE(column_name)

For Example: To create an employee table with Unique key, the query would be like,
Unique Key at column level:

CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10) UNIQUE
);


or

CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10) CONSTRAINT loc_un UNIQUE
);


Unique Key at table level:

CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
salary number(10),
location char(10),
CONSTRAINT loc_un UNIQUE(location)
);



5) check


This constraint defines a business rule on a column. All the rows must satisfy this rule. The constraint can be applied for a single column or a group of columns.
Syntax to define a Check constraint:

[CONSTRAINT constraint_name] CHECK (condition)

For Example: In the employee table to select the gender of a person, the query would be like
Check Constraint at column level:

CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
gender char(1) CHECK (gender in ('M','F')),
salary number(10),
location char(10)
);


Check Constraint at table level:
CREATE TABLE employee
( id number(5) PRIMARY KEY,
name char(20),
dept char(10),
age number(2),
gender char(1),
salary number(10),
location char(10),
CONSTRAINT gender_ck CHECK (gender in ('M','F'))
);