UNION用于把来自许多SELECT语句的结果组合到一个结果集合中，也叫联合查询。
SELECT ... UNION [ALL | DISTINCT] SELECT ... [UNION [ALL | DISTINCT] SELECT ...]
在多个 SELECT 语句中，第一个 SELECT 语句中被使用的字段名称将被用于结果的字段名称。
当使用 UNION 时，MySQL 会把结果集中重复的记录删掉，而使用 UNION ALL ，MySQL 会把所有的记录返回，且效率高于 UNION。
数据准备
student表
-- ----------------------------
-- Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(10) DEFAULT NULL,
`age` tinyint(4) DEFAULT NULL,
`classId` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 's1', '20', '1');
INSERT INTO `student` VALUES ('2', 's2', '22', '1');
INSERT INTO `student` VALUES ('3', 's3', '22', '2');
INSERT INTO `student` VALUES ('4', 's4', '25', '2');
teacher表
-- ----------------------------
-- Table structure for `teacher`
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(10) DEFAULT NULL,
`age` tinyint(4) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of teacher
-- ----------------------------
INSERT INTO `teacher` VALUES ('1', 't1', '36');
INSERT INTO `teacher` VALUES ('2', 't2', '33');
INSERT INTO `teacher` VALUES ('3', 's3', '22');
查询数据如下
mysql> SELECT * FROM student; +----+------+-----+---------+ | id | name | age | classId | +----+------+-----+---------+ | 1 | s1 | 20 | 1 | | 2 | s2 | 22 | 1 | | 3 | s3 | 22 | 2 | | 4 | s4 | 25 | 2 | +----+------+-----+---------+ 4 rows in set mysql> SELECT * FROM teacher; +----+------+-----+ | id | name | age | +----+------+-----+ | 1 | t1 | 36 | | 2 | t2 | 33 | | 3 | s3 | 22 | +----+------+-----+ 3 rows in set
使用 UNION的结果
mysql> SELECT id, name, age FROM student
-> UNION -- 与UNION DISTINCT相同
-> SELECT id, name, age FROM teacher;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | s1 | 20 |
| 2 | s2 | 22 |
| 3 | s3 | 22 |
| 4 | s4 | 25 |
| 1 | t1 | 36 |
| 2 | t2 | 33 |
+----+------+-----+
6 rows in set
使用 UNION ALL的结果
mysql> SELECT id, name, age FROM student
-> UNION ALL
-> SELECT id, name, age FROM teacher;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | s1 | 20 |
| 2 | s2 | 22 |
| 3 | s3 | 22 |
| 4 | s4 | 25 |
| 1 | t1 | 36 |
| 2 | t2 | 33 |
| 3 | s3 | 22 |
+----+------+-----+
7 rows in set
其实联合查询跟字段的类型无关，只要求每个SELECT查询的字段数一样，能对应即可，如
mysql> SELECT id, name, age FROM student -- 这里可以看出第一个SELECT语句中的字段名称被用作最后结果的字段名
-> UNION
-> SELECT age, name, id FROM teacher;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | s1 | 20 |
| 2 | s2 | 22 |
| 3 | s3 | 22 |
| 4 | s4 | 25 |
| 36 | t1 | 1 |
| 33 | t2 | 2 |
| 22 | s3 | 3 |
+----+------+-----+
7 rows in set
在联合查询中，当使用ORDER BY的时候，需要对SELECT语句添加括号，并且与LIMIT结合使用才生效，如
mysql> (SELECT classId, id, name, age FROM student WHERE classId = 1 ORDER BY age DESC)
-> UNION
-> (SELECT classId, id, name, age FROM student WHERE classId = 2 ORDER BY age);
+---------+----+------+-----+
| classId | id | name | age |
+---------+----+------+-----+
| 1 | 1 | s1 | 20 |
| 1 | 2 | s2 | 22 |
| 2 | 3 | s3 | 22 |
| 2 | 4 | s4 | 25 |
+---------+----+------+-----+
4 rows in set
此时classId为1的学生并没有按照年龄进行降序，结合LIMIT后
mysql> (SELECT classId, id, name, age FROM student WHERE classId = 1 ORDER BY age DESC LIMIT 2)
-> UNION
-> (SELECT classId, id, name, age FROM student WHERE classId = 2 ORDER BY age);
+---------+----+------+-----+
| classId | id | name | age |
+---------+----+------+-----+
| 1 | 2 | s2 | 22 |
| 1 | 1 | s1 | 20 |
| 2 | 3 | s3 | 22 |
| 2 | 4 | s4 | 25 |
+---------+----+------+-----+
4 rows in set
原创文章，作者：Maggie-Hunter，如若转载，请注明出处：https://blog.ytso.com/4795.html