UNIQUE CONSTRAINT in Oracle SQL – PLSQL

In Oracle SQL / PLSQL a UNIQUE CONSTRAINT is single or group of column(s)/field(s) that can uniquely identify each record in a table.

Some of the column(s) / field(s) of unique constraint can have NULL values until the combination can uniquely identify each record of table.

Important points about UNIQUE CONSTRAINT in Oracle SQL / PLSQL:

1. In Oracle SQL / PLSQL a UNIQUE CONSTRAINT can be created by using maximum of 32 columns / fields.
2. A UNIQUE CONSTRAINT can be defined in CREATE TABLE or ALTER TABLE statement.

Syntax to create a UNIQUE CONSTRAINT in CREATE TABLE statement is:

CREATE TABLE table_name
(column_name1 datatype NULL/NOT NULL
,column_name2 datatype NULL/NOT NULL
,column_name3 datatype NULL/NOT NULL
.
.
,column_nameN datatype NULL/NOT NULL
,CONSTRAINT constraint_name UNIQUE (column_name1, column_name2, . . . ,column_nameN );

Syntax to create a UNIQUE CONSTRAINT in ALTER TABLE statement is:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column_name1, column_name2, . . . ,column_nameN );

Let’s take an example for understanding:

Scenario 1:

Step 1: Suppose we want to create a table named ‘employee’ in the database as shown below, with ‘employee_id’ column having UNIQUE CONSTRAINT.

Employee_Id	Employee_Name	Salary	Department	Commission
101	Emp A	10000	Sales	10
102	Emp B	20000	IT	20
103	Emp C	28000	IT	20
104	Emp D	30000	Support
105	Emp E	32000	Sales	10

We can achieve the same as:

CREATE TABLE employee
(employee_id   NUMBER(10)      NOT NULL
,employee_name VARCHAR2(500)   NOT NULL
,salary        NUMBER(20)      NOT NULL
,department    VARCHAR2(300)   NOT NULL
,commission    NUMBER(20)
,CONSTRAINT employee_pk UNIQUE (employee_id)
);

Here with the help of the above SQL CREATE statement we have created a table named ‘employee’ that has 5 columns namely ‘employee_id’, ‘employee_name’, ‘department’, ‘salary’ and ‘commission’. Also we are having UNIQUE CONSTRAINT on ‘employee_id’ column of the ‘employee’ table.

Step 2: Inserting the data in the ‘employee’ table.

INSERT INTO employee
VALUES (101,'Emp A',10000,'Sales',10);

INSERT INTO employee
VALUES (102,'Emp B',20000,'IT',20);

INSERT INTO employee
VALUES (103,'Emp C',28000,'IT',20);

INSERT INTO employee
VALUES (104,'Emp D',30000,'Support',NULL);

INSERT INTO employee
VALUES (105,'Emp E',32000,'Sales',10);

The above SQL INSERT statements will insert 5 rows in the ‘employee’ table.

Now, if we query the employee table as:

SELECT *
FROM employee;

We will get the following result:

Employee_Id	Employee_Name	Salary	Department	Commission
101	Emp A	10000	Sales	10
102	Emp B	20000	IT	20
103	Emp C	28000	IT	20
104	Emp D	30000	Support
105	Emp E	32000	Sales	10

Here we have successfully created ‘employee’ table with column ‘employee_id’ having UNIQUE CONSTRAINT and also inserted data in ‘employee’ table.

If we try to insert one more record of employee_id = 105 as:

INSERT INTO employee
VALUES (105,'Emp F',40000,'Sales',30);

We get an error that unique constraint is getting violated and the data cannot be entered, this is because ‘employee_id’ column has UNIQUE CONSTRAINT and for employee_id = 105 we already have a record in ‘employee’ table and hence one more record of the same key cannot be entered.

Scenario 2:

Creating a combination of columns as UNIQUE CONSTRAINT

Suppose we want to create a table named ‘employee’ in the database as shown below, with combination of ‘employee_id’ and ‘employee_name’ as the UNIQUE CONSTRAINT columns for the ‘employee’ table

Employee_Id	Employee_Name	Salary	Department	Commission
101	Emp A	10000	Sales	10
102	Emp B	20000	IT	20
103	Emp C	28000	IT	20
104	Emp D	30000	Support
105	Emp E	32000	Sales	10

We can achieve the same as:

CREATE TABLE employee
(employee_id   NUMBER(10)       NOT NULL
,employee_name VARCHAR2(500)    NOT NULL
,salary        NUMBER(20)       NOT NULL
,department    VARCHAR2(300)    NOT NULL
,commission    NUMBER(20)
,CONSTRAINT employee_pk UNIQUE (employee_id,employee_name)
);

Let’s insert some data in the newly created ‘employee’ table

INSERT INTO employee
VALUES (101,'Emp A',10000,'Sales',10);

INSERT INTO employee
VALUES (102,'Emp B',20000,'IT',20);

INSERT INTO employee
VALUES (103,'Emp C',28000,'IT',20);

INSERT INTO employee
VALUES (104,'Emp D',30000,'Support',NULL);

INSERT INTO employee
VALUES (105,'Emp E',32000,'Sales',10);

The above SQL INSERT statements will insert 5 rows in the ‘employee’ table.

Now, if we query the employee table as:

SELECT *
FROM employee;

We will get the following result:

Employee_Id	Employee_Name	Salary	Department	Commission
101	Emp A	10000	Sales	10
102	Emp B	20000	IT	20
103	Emp C	28000	IT	20
104	Emp D	30000	Support
105	Emp E	32000	Sales	10

Suppose we want to insert one more record with ‘employee_id = 105’ and ‘employee_name = Emp F’ then we can do this and oracle will not give any error because the combination of ‘employee_id’ and ‘employee_name’ is still unique.

Scenario 3:

Using ALTER TABLE statement to create UNIQUE CONSTRAINT on a table

As mentioned earlier that an ALTER TABLE statement can be used to create UNIQUE CONSTRAINT on a table.
The syntax to create UNIQUE CONSTRAINT using ALTER TABLE statement is:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column_name1,column_name2, . . column_nameN);

Example:

ALTER TABLE employee
ADD CONSTRAINT employee_pk UNIQUE (employee_id);

Here we have altered the ‘employee’ table and have successfully created UNIQUE CONSTRAINT on ‘employee_id’ column.

We can also create UNIQUE CONSTRAINT by a combination of columns using the ALTER TABLE statement

Example:

ALTER TABLE employee
ADD CONSTRAINT employee_pk UNIQUE (employee_id, employee_name);

Here we have altered the ‘employee’ table and have successfully created UNIQUE CONSTRAINT on ‘employee_id’ and ‘employee_name’ column.