SQL DELETE DUPLICATE ROWS
When dealing with large datasets, it’s common to encounter duplicate records in a table. Duplicate data can cause inaccurate record fetching results if it is not managed appropriately. To ensure accuracy, we must retrieve unique records instead of duplicates.
To solve this issue, SQL provides the DISTINCT keyword, which is used with the SELECT statement to eliminate duplicate records and fetch only unique ones.
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Using DISTINCT to Remove Duplicates
The DISTINCT keyword helps retrieve unique values from a specific column or a combination of columns. Below is the syntax:
SELECT DISTINCT column1, column2, ....columnN
FROM table_name
WHERE [conditions];
Example
Let’s consider a EMPLOYEE table with duplicate salary records.
| EMP_ID | NAME | SALARY | CITY |
|---|---|---|---|
| 101 | RAHUL VERMA | 50000 | MUMBAI |
| 102 | NEHA SINGH | 48000 | DELHI |
| 103 | ARJUN SHARMA | 50000 | BANGALORE |
| 104 | SAKSHI JAIN | 52000 | KOLKATA |
| 105 | ROHAN MEHTA | 48000 | PUNE |
| 106 | KRITI AGGARWAL | 51000 | CHENNAI |
Checking for Duplicate Salary Records
Before applying DISTINCT, let’s retrieve all salary records to check for duplicates:
SELECT SALARY FROM EMPLOYEE
ORDER BY SALARY;
Output:
| SALARY |
|---|
| 48000 |
| 48000 |
| 50000 |
| 50000 |
| 51000 |
| 52000 |
Using DISTINCT to Remove Duplicates
Now, we apply the DISTINCT keyword to fetch unique salary records:
SELECT DISTINCT SALARY FROM EMPLOYEE
ORDER BY SALARY;
Output:
| SALARY |
|---|
| 48000 |
| 50000 |
| 51000 |
| 52000 |
Duplicate salary values have been removed, as can be seen above.
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Conclusion
In order to preserve data integrity, SQL must handle duplicate records. The DISTINCT keyword is a simple yet powerful solution to fetch unique records from a table, ensuring accurate and meaningful data retrieval.
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