MySQL利用UNION连接2个查询排序失效详解

-- 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');

-- 查询1
SELECT
 *
FROM
 test_user u
ORDER BY AGE

-- 查询2
-- 使用UNION
(
 SELECT
  *
 FROM
  test_user u
 ORDER BY AGE
)
UNION
(
 SELECT
  *
 FROM
  test_user u
 ORDER BY AGE
);
 
-- 查询3
-- 使用UNION ALL
(
 SELECT
  *
 FROM
  test_user u
 ORDER BY AGE
)
UNION ALL
(
 SELECT
  *
 FROM
  test_user u
 ORDER BY AGE
)

-- 查询4
-- 将UNION ALL作为子查询并进行排序
SELECT
 *
FROM
 (
 (
 SELECT
 *
 FROM
 test_user u
 ORDER BY
 AGE
 )
 UNION ALL
 (
 SELECT
 *
 FROM
 test_user u
 ORDER BY
 AGE
 )
 ) a
ORDER BY
 AGE;

-- 查询5
-- 第一个查询不使用排序,如果使用的话不加括号会报错(这也是我之前为什么会想用子查询的原因而没有想过这种方式了)
SELECT
 *
FROM
 test_user u
 
UNION ALL
 
SELECT
 *
FROM
 test_user u
ORDER BY
 AGE