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)