MySQL——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

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;


INSERT INTO `student` VALUES ('1', 's1', '20', '1'), ('2', 's2', '22', '1'),('3', 's3', '22', '2'), ('4', 's4', '25', '2');

teacher表

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;
 

INSERT INTO `teacher` VALUES ('1', 't1', '36'), ('2', 't2', '33'), ('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