-- 創建表
CREATE TABLE test_user (
 ID int(11) NOT NULL AUTO_INCREMENT,
 USER_ID int(11) DEFAULT NULL COMMENT '用戶賬號',
 USER_NAME varchar(255) DEFAULT NULL COMMENT '用戶名',
 AGE int(5) DEFAULT NULL COMMENT '年齡',
 COMMENT varchar(255) DEFAULT NULL COMMENT '簡介',
 PRIMARY KEY (ID)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
 
-- 數據插入語句
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('1', '111', '開心菜鳥', '18', '今天很開心');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('2', '222', '悲傷菜鳥', '21', '今天很悲傷');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('3', '333', '認真菜鳥', '30', '今天很認真');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('4', '444', '高興菜鳥', '18', '今天很高興');
INSERT INTO test_user (ID, USER_ID, USER_NAME, AGE, COMMENT) VALUES ('5', '555', '嚴肅菜鳥', '21', '今天很嚴肅');

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

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

-- 查詢4
-- 將UNION ALL作為子查詢并進行排序
SELECT
 *
FROM
 (
 (
 SELECT
 *
 FROM
 test_user u
 ORDER BY
 AGE
 )
 UNION ALL
 (
 SELECT
 *
 FROM
 test_user u
 ORDER BY
 AGE
 )
 ) a
ORDER BY
 AGE;

-- 查詢5
-- 第一個查詢不使用排序,如果使用的話不加括號會報錯(這也是我之前為什么會想用子查詢的原因而沒有想過這種方式了)
SELECT
 *
FROM
 test_user u
 
UNION ALL
 
SELECT
 *
FROM
 test_user u
ORDER BY
 AGE