UNION

The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.
UNION命令可以用来选择两个有关联的信息，和JOIN命令非常相似。然而当使用UNION命令时得保证所选择的栏目数据类型相同。

Note: With UNION, only distinct values are selected.
注意：使用UNION，只有不同的值会被选择出来。

SQL Statement 1
UNION
SQL Statement 2

Employees_Norway表:

Employees_USA表:

Using the UNION Command
使用UNION 命令

Example
举例

List all different employee names in Norway and USA:
列举在USA和Norway中不同的人员名字：

SELECT E_Name FROM Employees_Norway
UNION
SELECT E_Name FROM Employees_USA

Result
结果

Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them is listed. The UNION command only selects distinct values.
注意：这个命令不能将Norway和USA中所有的人员列举出来。在上面的举例中两个表有相同人员名字的数据，最后列出来的只会是其中的一个。

UNION ALL

The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
UNION ALL命令等同于UNION命令，但UNION ALL会选择全部的值

SQL Statement 1
UNION ALL
SQL Statement 2

Using the UNION ALL Command
使用 UNION ALL命令

Example举例

List all employees in Norway and USA:
列举出在Norway和USA中所有的员工：

SELECT E_Name FROM Employees_Norway
UNION ALL
SELECT E_Name FROM Employees_USA

Result
结果

SQL Join SQL Create

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