Understanding of the Union operation in SQL

In SQL, for and operations, you can use the Union keyword.

For example:

SELECT  from table_name1 UNION SELECT  from Table_name2

During the learning process, I had a question about what would happen if the property names of the two tables were different.

Directly simulating an example, the build code is as follows:

Create TableEmployee (EmpNameChar(6), Numchildrenint)Create TableDependent (DepnameChar(6), ageint)Insert  intoEmployeeValues('Haohao',2);Insert  intoEmployeeValues('Haohao',4);Insert  intoEmployeeValues('FGSD',5);Insert  intoEmployeeValues('HDS',7);Insert  intoEmployeeValues('Hauuo',9);Insert  intoEmployeeValues('Hsao',4);Insert  intoEmployeeValues('Hhao',5);Insert  intoDependentValues('Haohao',2);Insert  intoDependentValues('HDGSO',2);Insert  intoDependentValues('Hreo',2);Insert  intoDependentValues('Hjh',2);Insert  intoDependentValues('Haaao',2);

The properties of the two tables are just the same as the data types, and the test results are as follows:

(Select  empname,numchildrenfrom  employee)Union(  Select  depname,age from dependent)

The result is:

"Hjh"; 2
"Hhao"; 5
"Haohao"; 2
"HDGSO"; 2
"Haohao"; 4
"FGSD"; 5
"HDS"; 7
"Hsao"; 4
"Haaao"; 2
"Hreo"; 2
"Hauuo"; 9

As can be seen, the merger was carried out, and the duplication was removed.

Later, the SQL module in W3school saw the definition for the and operation:

Note that the SELECT statement inside the UNION must have the same number of columns. The column must also have a similar data type. Also, the order of the columns in each SELECT statement must be the same. ”

Reference Link: http://www.w3school.com.cn/sql/sql_union.asp

2015-05-20

17:58:35

Understanding of the Union operation in SQL