14.UNION 和 UNION ALL 操作符

union all   union

UNION 操作符用于合并两个或多个 SELECT 语句的结果集。
请注意，UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每条 SELECT 语句中的列的顺序必须相同。

CREATE TABLE Employees_China(
   E_ID   INT              NOT NULL,
    E_Name  CHAR (25)              NOT NULL
);
 
CREATE TABLE Employees_USA(
   E_ID   INT              NOT NULL,
    E_Name  CHAR (25)              NOT NULL
);

INSERT INTO Employees_China (E_ID,E_Name) VALUES(1,'Zhang, Hua');
INSERT INTO Employees_China (E_ID,E_Name) VALUES(2, 'Wang, Wei');
INSERT INTO Employees_China (E_ID,E_Name) VALUES(3,'Carter, Thomas');
INSERT INTO Employees_China (E_ID,E_Name) VALUES(4,'Yang, Ming');
INSERT INTO Employees_USA (E_ID,E_Name) VALUES(1,'Adams, John');
INSERT INTO Employees_USA (E_ID,E_Name) VALUES(2,'Bush, George');
INSERT INTO Employees_USA (E_ID,E_Name) VALUES(3,'Carter, Thomas');
INSERT INTO Employees_USA (E_ID,E_Name) VALUES(4,'Gates, Bill');

mysql> select * from Employees_China;
+------+----------------+
| E_ID | E_Name         |
+------+----------------+
|    1 | Zhang, Hua     |
|    2 | Wang, Wei      |
|    3 | Carter, Thomas |
|    4 | Yang, Ming     |
+------+----------------+
4 rows in set (0.06 sec)

mysql> select * from Employees_USA;
+------+----------------+
| E_ID | E_Name         |
+------+----------------+
|    1 | Adams, John    |
|    2 | Bush, George   |
|    3 | Carter, Thomas |
|    4 | Gates, Bill    |
+------+----------------+
4 rows in set (0.00 sec)
 
1.
mysql> SELECT E_Name FROM Employees_China
    -> UNION
    -> SELECT E_Name FROM Employees_USA;
+----------------+
| E_Name         |
+----------------+
| Zhang, Hua     |
| Wang, Wei      |
| Carter, Thomas |
| Yang, Ming     |
| Adams, John    |
| Bush, George   |
| Gates, Bill    |
+----------------+
7 rows in set (0.06 sec)
注释：默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。

2.
mysql> SELECT E_Name FROM Employees_China
    -> UNION ALL
    -> SELECT E_Name FROM Employees_USA;
+----------------+
| E_Name         |
+----------------+
| Zhang, Hua     |
| Wang, Wei      |
| Carter, Thomas |
| Yang, Ming     |
| Adams, John    |
| Bush, George   |
| Carter, Thomas |
| Gates, Bill    |
+----------------+
8 rows in set (0.00 sec)

另外，UNION 结果集中的列名总是等于 UNION 中第一个 SELECT 语句中的列名。