UNION operator in SQL Server

The UNION operator is used to combine 2 result sets from 2 or more SELECT statements. It will delete the same rows in these SELECT statements.

Each SELECT in the UNION operator must have the same number of columns in the result set with the corresponding data type.

The UNION operator syntax

 SE LECT bieu_thuc1, bieu_thuc2, … bieu_thucn 
 FROM bang 
 [WHERE dieu_kien] 
 UNION 
 SELECT bieu_thuc1, bieu_thuc2, … bieu_thucn 
 FROM bang 
 [WHERE dieu_kien]; 

Variable name or variable value

bieu_thuc1, bieu_thuc2, . bieu_thucn

The column or calculation value you want to retrieve.

state

Table wants to get the record. Must have at least 1 table in the FROM clause.

WHERE dieu_kien

Option. Conditions must satisfy for the selected record.

Note:

1. 2 SELECT statements must have the same number of expressions
2. The corresponding number of columns in each SELECT statement must have the same data type
3. UNION operator clears duplicate rows.
4. 
   See more UNION ALL operators.

For example - return an information field

For example, the UNION operator returns a field from multiple SELECT statements (and fields with the same data type).

 SELECT san pham_id 
 FROM sanpham 
 UNION 
 SELECT sanpham_id 
 FROM hangtonkho; 

In the above example, if any sanpham_id appears in both the sanpham and hangtonkho tables, only 1 sanpham_id will appear in the result set. If you do not want to remove the duplicate record, use the UNION ALL operator.

For example - use ORDER BY

The UNION operator can use the ORDER BY clause to sort query results.

 SELE CT danhba_id, danhba_ten 
 FROM danhba 
 WHERE ten_trang = 'QuanTriMang.com' 
 UNION 
 SELECT congty_id, congty_ten 
 FROM congty 
 WHERE ten_trang = 'TrangCuaBan.com' 
 ORDER B Y 2; 

In this example, because the column name in the two SELECT statements is different, it is easier to refer to the column in the ORDER BY statement by position in the result set. In the above example, we filter the results by namba_ten / congty_ten in ascending order, as stated by the phrase ORDER BY 2.

danhba_ten / congty_ten is in the second position in the result set.

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