PostgreSQL - DISTINCT Keyword

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The PostgreSQL DISTINCT keyword is used in conjunction with SELECT statement to eliminate all the duplicate records and fetching only unique records.

There may be a situation when you have multiple duplicate records in a table. While fetching such records, it makes more sense to fetch only unique records instead of fetching duplicate records.

Syntax:

The basic syntax of DISTINCT keyword to eliminate duplicate records is as follows:

SELECT DISTINCT column1, column2,.....columnN
FROM table_name
WHERE [condition]

Example:

Consider the table COMPANY having records as follows:

# select * from COMPANY;
 id | name  | age | address   | salary
----+-------+-----+-----------+--------
  1 | Paul  |  32 | California|  20000
  2 | Allen |  25 | Texas     |  15000
  3 | Teddy |  23 | Norway    |  20000
  4 | Mark  |  25 | Rich-Mond |  65000
  5 | David |  27 | Texas     |  85000
  6 | Kim   |  22 | South-Hall|  45000
  7 | James |  24 | Houston   |  10000
(7 rows)

Let us add two more records to this table as follows:

INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (8, 'Paul', 32, 'California', 20000.00 );

INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (9, 'Allen', 25, 'Texas', 15000.00 );

Now, the records in the COMPANY table would be:

 id | name  | age | address    | salary
----+-------+-----+------------+--------
  1 | Paul  |  32 | California |  20000
  2 | Allen |  25 | Texas      |  15000
  3 | Teddy |  23 | Norway     |  20000
  4 | Mark  |  25 | Rich-Mond  |  65000
  5 | David |  27 | Texas      |  85000
  6 | Kim   |  22 | South-Hall |  45000
  7 | James |  24 | Houston    |  10000
  8 | Paul  |  32 | California |  20000
  9 | Allen |  25 | Texas      |  15000
(9 rows)

First, let us see how the following SELECT query returns duplicate salary records:

testdb=# SELECT name FROM COMPANY;

This would produce the following result:

 name
-------
 Paul
 Allen
 Teddy
 Mark
 David
 Kim
 James
 Paul
 Allen
(9 rows)

Now, let us use DISTINCT keyword with the above SELECT query and see the result:

testdb=# SELECT DISTINCT name FROM COMPANY;

This would produce the following result where we do not have any duplicate entry:

 name
-------
 Teddy
 Paul
 Mark
 David
 Allen
 Kim
 James
(7 rows)

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