SQL - Handling Duplicates

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 SQL DISTINCT keyword, which we already have discussed, is used in conjunction with SELECT statement to eliminate all the duplicate records and fetching only unique 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 CUSTOMERS table having the following records:

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 | 

 +----+----------+-----+-----------+----------+

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

SQL> SELECT SALARY FROM CUSTOMERS
     ORDER BY SALARY;

This would produce the following result where salary 2000 is coming twice which is a duplicate record from the original table.

+----------+
| SALARY   |
+----------+
|  1500.00 |
|  2000.00 |
|  2000.00 |
|  4500.00 |
|  6500.00 |
|  8500.00 |
| 10000.00 |
+----------+

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

SQL> SELECT DISTINCT SALARY FROM CUSTOMERS
     ORDER BY SALARY;

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

+----------+
| SALARY   |
+----------+
|  1500.00 |
|  2000.00 |
|  4500.00 |
|  6500.00 |
|  8500.00 |
| 10000.00 |
+----------+