MySQL学习-- UNION与UNION ALL
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。
1 2 3 4 5 | 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | -- ------------------------------ 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | -- ------------------------------ 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | 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 setmysql> 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | 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
1 2 3 4 5 6 7 8 9 10 11 12 | 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
1 2 3 4 5 6 7 8 9 10 11 12 | 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 |
