MySQL中UNION语句用法详解与示例

-- 创建表
CREATE TABLE test_user (
 ID int(11) NOT NULL AUTO_INCREMENT,
 USER_ID int(11) DEFAULT NULL COMMENT '用户账号',
 USER_NAME varchar(255) DEFAULT NULL COMMENT '用户名',
 AGE int(5) DEFAULT NULL COMMENT '年龄',
 COMMENT varchar(255) DEFAULT NULL COMMENT '简介',
 PRIMARY KEY (ID)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
-- 数据插入语句
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('1', '111', '开心菜鸟', '18', '今天很开心');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('2', '222', '悲伤菜鸟', '21', '今天很悲伤');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('3', '333', '认真菜鸟', '30', '今天很认真');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('4', '444', '高兴菜鸟', '18', '今天很高兴');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('5', '555', '严肃菜鸟', '21', '今天很严肃');

SELECT * FROM test_user u;

-- 使用UNION
SELECT * FROM test_user u
UNION
SELECT * FROM test_user u;

-- 使用UNION ALL
SELECT * FROM test_user u
UNION ALL
SELECT * FROM test_user u;

-- 1、第二个子句中的 where 语句不能同时作用于两个select语句
-- 5 + 1，共计 6 条
SELECT *, 'table1' FROM test_user u 
UNION ALL
SELECT *, 'table2' FROM test_user u WHERE AGE = 30;

-- 1、第二个子句中的 where 语句不能同时作用于两个select语句
-- 1 + 1，共计 2 条
-- 写法 1
SELECT * FROM 
(
	SELECT *, 'table1' FROM test_user u 
	UNION ALL
	SELECT *, 'table2' FROM test_user u
) a 
WHERE AGE = 30;
-- 或者使用写法 2
SELECT *, 'table1' FROM test_user u WHERE AGE = 30
UNION ALL
SELECT *, 'table2' FROM test_user u WHERE AGE = 30
;

-- 2、第二个子句中的 group by 语句不能同时作用于两个select语句
-- 5 + 3，共计 8 条
SELECT *, 'table1' FROM test_user u 
UNION ALL
SELECT *, 'table2' FROM test_user u GROUP BY AGE;

-- 3、第二个子句中的 HAVING 语句不能同时作用于两个 select 语句
-- 5 + 1，共计 6 条
SELECT *, 'table1' FROM test_user u 
UNION ALL
SELECT *, 'table2' FROM test_user u HAVING AGE = 30 ;

-- 4、第二个子句中的 order by 语句可以同时作用于两个select语句 
-- 查询结果整体按照 age 进行了排序
SELECT *, 'table1' FROM test_user u 
UNION ALL
SELECT *, 'table2' FROM test_user u ORDER BY AGE;

-- 只有1条数据，因为LIMIT在UNION之后执行
SELECT *, 'table1' FROM test_user u 
UNION ALL
SELECT *, 'table2' FROM test_user u limit 0,1;

-- 5、第二个子句中的 order by ,LIMIT 语句同时作用于两个 select 语句 *********
-- 只有1条数据，age=30 UNION--->ORDER BY--->LIMIT
SELECT *, 'table1' FROM test_user u 
UNION ALL
SELECT *, 'table2' FROM test_user u order by age desc limit 0,1;

SELECT * FROM test_user u ORDER BY AGE;

SELECT * FROM test_user u ORDER BY AGE DESC;

(SELECT *, 'table1' FROM test_user u ORDER BY AGE) 
UNION ALL 
(SELECT *, 'table2' FROM test_user u ORDER BY AGE DESC);

-- 都加上 LIMIT
( SELECT *, 'table1' FROM test_user u ORDER BY AGE limit 10) 
UNION ALL 
( SELECT *, 'table2' FROM test_user u ORDER BY AGE DESC limit 10) 

select * from 
(
	( SELECT *, 'table1' AS name, row_number() over(ORDER BY AGE ) AS rn FROM test_user u ) 
	UNION ALL 
	( SELECT *, 'table2' AS name, row_number() over(ORDER BY AGE DESC) AS rn FROM test_user u ) 
) a
order by name, rn;

-- 语法错误
SELECT *, 'table1' FROM test_user u ORDER BY AGE
UNION ALL
SELECT *, 'table2' FROM test_user u ORDER BY AGE DESC;

-- 语法正确
(SELECT *, 'table1' FROM test_user u ORDER BY AGE)
UNION ALL
SELECT *, 'table2' FROM test_user u ORDER BY AGE DESC;

-- 语法正确
(SELECT *, 'table1' FROM test_user u ORDER BY AGE)
UNION ALL
(SELECT *, 'table2' FROM test_user u ORDER BY AGE DESC);

-- 语法错误
SELECT *, 'table1' FROM test_user u limit 0,1
UNION ALL
SELECT *, 'table2' FROM test_user u limit 0,1;

-- 语法正确，1 条记录
(SELECT *, 'table1' FROM test_user u limit 0,1)
UNION ALL
SELECT *, 'table2' FROM test_user u limit 0,1;

-- 语法正确，2 条记录
(SELECT *, 'table1' FROM test_user u limit 0,1)
UNION ALL
(SELECT *, 'table2' FROM test_user u limit 0,1);