DISTINCT Keyword in SQL

This guide covers distinct keyword in SQL with practical context and easy-to-follow details. Use it to understand the subject and apply the information confidently.

Syntax to Use DISTINCT in SQL

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

SELECT DISTINCT cot1, cot2,. cotN
FROM ten_bang
WHERE [dieu_kien]

Example of DISTINCT in SQL

Suppose the NHANVIEN table has the following records:

 +----+----------+-----+-----------+----------+ | ID | TEN |TUOI | DIACHI | LUONG | +----+----------+-----+-----------+----------+ | 1 | Thanh | 32 | Haiphong | 2000.00 | | 2 | Loan | 25 | Hanoi | 1500.00 | | 3 | Nga | 23 | Hanam | 2000.00 | | 4 | Manh | 25 | Hue | 6500.00 | | 5 | Huy | 27 | Hatinh | 8500.00 | | 6 | Cao | 22 | HCM | 4500.00 | | 7 | Lam | 24 | Hanoi | 10000.00 | +----+----------+-----+-----------+----------+

Start by let's see how the SELECT query returns the duplicate salary record.

 SQL> SELECT LUONG FROM NHANVIEN
 ORDER BY LUONG;

In the following result, LUONG 2000 appears twice as a duplicate record from the original table.

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

Now, use the DISTINCT keyword with the SELECT query and see the result.

SQL> SELECT DISTINCT LUONG FROM NHANVIEN
 ORDER BY LUONG;

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

Thanks to DISTINCT, the results obtained do not have any duplicate entries.

In the next section, we will learn about sorting results in SQL, remember to follow them.

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