描述

用来将多路相同schema的数据做合并。要求两边的字段完全一致，包括字段类型、字段顺序。

UNION ALL 会保留重复值，而UNION 会进行去重处理，等价于UNION ALL + DISTINCT，运行效率较低，不建议使用。

多路UNION时，不支持UNION和UNION ALL混用；

语法格式

query:
   select_statement
	UNION [ALL]
	select_statement
   [UNION [ALL]
	select_statement];

示例

双路合并：

SELECT nid, brand, price, size FROM phone WHERE nid < 5
UNION ALL
SELECT nid, brand, price, size FROM phone WHERE nid > 5

多路合并：

SELECT 
    nid, brand, price, size 
FROM 
    phone 
WHERE nid >= 3 AND nid <= 5 
UNION ALL 
SELECT 
    nid, brand, price, size 
FROM 
    phone 
WHERE nid >= 6 AND nid <= 10 
UNION ALL 
SELECT nid, brand, price, size 
FROM 
    phone 
WHERE nid >= 30 AND nid <= 40