DISTINCT keyword in SQL

In SQL, the DISTINCT keyword is used in conjunction with the SELECT statement to remove all duplicate records and retrieve only records in the table.

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 | +----+----------+-----+-----------+----------+

First, 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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