MySQL学习(五) 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。
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