The Data validation is always a major concern of any Database System.
We all know about the UNIQUE constraint which prevents the duplicate records.

One of the Previous article, I have already shared one small note about the UNIQUE Index of PostgreSQL.

PostgreSQL: Composite UNIQUE Constraint does not consider NULL Values

In this post, I am sharing a similar post for Partial Unique Index of PostgreSQL.

For example, I want to stop duplicate Student Grages for particular Student Division only.

Create a sample table:

CREATE TABLE tbl_Students
(
	StudID INT 
	,StudName CHARACTER VARYING 
	,StudDivision CHARACTER VARYING
	,StudGrade CHAR(1)
);

Create a Partial UNIQUE Index:

CREATE UNIQUE INDEX tbl_Students_StudGrade ON tbl_Students (COALESCE(StudDivision,''),COALESCE(StudGrade,'')) 
WHERE StudDivision IN ('First','Second','Third');

Insert few sample records:
As we have creted Partial index so we can insert only unique combination of StudDivision (First,Second,Third) and StudGrade.

INSERT INTO tbl_Students
VALUES 
(1,'Anvesh','First','A')
,(2,'Jeeny','Second','B')
,(3,'Roy','Third','B')
,(4,'Mukesh','First','C')
,(5,'Martin','Second','C')
,(6,'Kavita','Third','A');

Try to insert few Invalid records:

INSERT INTO tbl_Students
VALUES 
(7,'Rajesh','First','A')
,(8,'Nile','Second','B');

Try to insert few records which are not validated by Partition Index:
We can insert duplicate records StudGrade + StudDivision for other StudDivisions except Partial StudDivision.

INSERT INTO tbl_Students
VALUES 
(7,'Dimpi','Fourth','C')
,(8,'Bony','Fourth','C');