PostgreSQL – DISTINCT Keyword
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.
The basic syntax of DISTINCT keyword to eliminate duplicate records is as follows −
SELECT DISTINCT column1, column2,.....columnN FROM table_name WHERE [condition]
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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