24、MySQL UNION 操作符查询多张表

MySQL UNION 操作符用于连接两个以上的 SELECT 语句的结果组合到一个结果集合中

多个SELECT 语句会删除重复的数据

UNION 操作符语法

MySQL UNION 操作符的语法格式如下

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION [ALL | DISTINCT]
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

参数 说明
expression1, expression2, … expression_n 要检索的列
tables 要检索的数据表
WHERE conditions 可选, 检索条件
DISTINCT 可选,删除结果集中重复的数据
默认情况下 UNION 操作符已经删除了重复数据
所以 DISTINCT 修饰符对结果没啥影响
ALL 可选,返回所有结果集,包含重复数据

范例数据

可以在mysql>` 命令行中运行以下语句填充范例数据

DROP TABLE IF EXISTS tbl_language;
DROP TABLE IF EXISTS tbl_rank;

CREATE TABLE IF NOT EXISTS tbl_language(
id INT UNSIGNED AUTO_INCREMENT,
name VARCHAR(64) NOT NULL,
url VARCHAR(128) NOT NULL,
founded_at DATE,
PRIMARY KEY ( id )
)ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE IF NOT EXISTS tbl_rank(
id INT UNSIGNED AUTO_INCREMENT,
name VARCHAR(64) NOT NULL,
month VARCHAR(7) NOT NULL,
rank TINYINT NOT NULL,
rate VARCHAR(32) NOT NULL,
PRIMARY KEY ( id )
)ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

INSERT INTO tbl_language VALUES
(1,'Python','https://ddkk.com','1991-2-20'),
(2,'PHP','http://www.php.net','1994-1-1'),
(3,'Ruby','https://www.ruby-lang.org/','1996-12-25');

INSERT INTO tbl_rank VALUES
(1, 'Python','2018-04',4,'5.083%'),
(2, 'PHP','2018-04',6,'4.218%'),
(3, 'Ruby','2018-04',11,'2.018%'),
(4, 'Java','2018-04',1,'15.777%'),
(5, 'Python','2018-03',4,'5.869%'),
(6, 'PHP','2018-03',7,'4.010%'),
(7, 'Ruby','2018-03',12,'2.744%'),
(8, 'Java','2018-03',1,'14.941'),
(9, 'Python','2018-02',4,'5.168%'),
(10, 'PHP','2018-02',7,'3.420%'),
(11, 'Ruby','2018-02',10,'2.534%'),
(12, 'Java','2018-02',1,'14.988%');

tbl_language 表中的数据如下

+----+--------+----------------------------+------------+
|id | name   | url                        | founded_at |
+----+--------+----------------------------+------------+
|1 | Python | https://ddkk.com        | 1991-02-20 |
|2 | PHP    | http://www.php.net         | 1994-01-01 |
|3 | Ruby   | https://www.ruby-lang.org/ | 1996-12-25 |
+----+--------+----------------------------+------------+

tbl_rank 表中的数据如下

+----+--------+---------+------+---------+
|id | name   | month   | rank | rate    |
+----+--------+---------+------+---------+
|1 | Python | 2018-04 |    4 | 5.083%  |
|2 | PHP    | 2018-04 |    6 | 4.218%  |
|3 | Ruby   | 2018-04 |   11 | 2.018%  |
|4 | Java   | 2018-04 |    1 | 15.777% |
|5 | Python | 2018-03 |    4 | 5.869%  |
|6 | PHP    | 2018-03 |    7 | 4.010%  |
|7 | Ruby   | 2018-03 |   12 | 2.744%  |
|8 | Java   | 2018-03 |    1 | 14.941  |
|9 | Python | 2018-02 |    4 | 5.168%  |
|10 | PHP    | 2018-02 |    7 | 3.420%  |
|11 | Ruby   | 2018-02 |   10 | 2.534%  |
|12 | Java   | 2018-02 |    1 | 14.988% |
+----+--------+---------+------+---------+

SQL UNION

下面的SQL 语句从 tbl_language 和 tbl_rank 表中选取所有 不同的 name(只有不同的值)

SELECT name FROM tbl_language UNION SELECT name FROM tbl_rank  ORDER BY name;

执行以上 SQL 输出结果如下

MariaDB [souyunku]> SELECT name FROM tbl_language UNION SELECT name FROM tbl_rank  ORDER BY name;
+--------+
|name   |
+--------+
|Java   |
|PHP    |
|Python |
|Ruby   |
+--------+
4rows in set (0.00 sec)

UNION 不能用于列出两个表中所有的 name ,UNION 只会选取不同的值

如果要选取重复的值,可以使用 UNION ALL

SQL UNION ALL

下面的SQL 语句从 tbl_language 和 tbl_rank 表中选取所有的 name(可能有重复的值)

SELECT name FROM tbl_language UNION ALL SELECT name FROM tbl_rank  ORDER BY name;

执行以上 SQL 输出结果如下

MariaDB [souyunku]> SELECT name FROM tbl_language UNION ALL SELECT name FROM tbl_rank  ORDER BY name;
+--------+
|name   |
+--------+
|Java   |
|Java   |
|Java   |
|PHP    |
|PHP    |
|PHP    |
|PHP    |
|Python |
|Python |
|Python |
|Python |
|Ruby   |
|Ruby   |
|Ruby   |
|Ruby   |
+--------+
15rows in set (0.00 sec)

带有 WHERE 的 SQL UNION ALL

下面的SQL 语句从 tbl_language 和 tbl_rank 表中选取所有的 Python(可能有重复的值)

SELECT name FROM tbl_language WHERE name="Python" UNION ALL SELECT name FROM tbl_rank WHERE name="Python"  ORDER BY name;

执行以上 SQL 输出结果如下

MariaDB [souyunku]> SELECT name FROM tbl_language WHERE name="Python" UNION ALL SELECT name FROM tbl_rank WHERE name="Python"  ORDER BY name;
+--------+
|name   |
+--------+
|Python |
|Python |
|Python |
|Python |
+--------+
4rows in set (0.00 sec)