9、多表查询-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_43605986/article/details/124117025

本文深入探讨了数据库查询的各种技术，包括子查询的四种用法，如比较表达式、IN操作、EXISTS条件及FROM子句中使用。接着，解释了联合查询如何实现多表纵向合并，以及交叉连接和内连接、左/右外连接的横向合并原理。最后，通过自连接展示了如何在同一张表中进行复杂的数据关联。这些知识对于理解和优化数据库查询性能至关重要。

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1、子查询

1.1、用于比较表达式中的子查询；子查询仅能返回单个值

[hellodb]> select name,age from students where age > (select avg(age) from teachers);
+-------------+-----+
| name        | age |
+-------------+-----+
| Sun Dasheng | 100 |
+-------------+-----+

1.2、用于IN中的子查询：子查询应该单独查询并返回一个或多个值重新构成列表

[hellodb]> select name,age from students where age in (select age from teachers);

1.3、exists：用在 where之后，且后面紧跟子查询语句，只关心子查询有没有结果,并不关心子查询的结果具体是什么

MariaDB [hellodb]> select * from students s where EXISTS (select * from teachers t where s.teacherid=t.tid);

1.4、用于FROM子句中的子查询

SELECT s.aage,s.ClassID FROM (SELECT avg(Age) AS aage,ClassID FROM students 
WHERE ClassID IS NOT NULL GROUP BY ClassID) AS s WHERE s.aage>30;

2、联合查询：多表纵向合并union，字段数量和属性要相匹配

MariaDB [hellodb]> select * from teachers union select * from teachers;

+-----+---------------+-----+--------+

| TID | Name | Age | Gender |

+-----+---------------+-----+--------+

| 1 | Song Jiang | 45 | M |

| 2 | Zhang Sanfeng | 94 | M |

| 3 | Miejue Shitai | 77 | F |

| 4 | Lin Chaoying | 93 | F |

+-----+---------------+-----+--------+

4 rows in set ( 0.00 sec)

3、交叉连接：横向合并，交叉连接（横向笛卡尔）

MariaDB [hellodb]> select * from students,teachers;
或者MariaDB [hellodb]> select * from students cross join teachers;

4、内连接：基于交叉连接对结果进行筛选 ,如果表定义了别名，原表名将无法使用

MariaDB [hellodb]> select * from students s inner join teachers t on s.teacherid=t.tid;
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name        | Age | Gender | ClassID | TeacherID | TID | Name          | Age | Gender |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
|     1 | Shi Zhongyu |  22 | M      |       2 |         3 |   3 | Miejue Shitai |  77 | F      |
|     4 | Ding Dian   |  32 | M      |       4 |         4 |   4 | Lin Chaoying  |  93 | F      |
|     5 | Yu Yutong   |  26 | M      |       3 |         1 |   1 | Song Jiang    |  45 | M      |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
3 rows in set (0.001 sec)

或者
MariaDB [hellodb]> select * from students s,teachers t where s.teacherid=t.tid;
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name        | Age | Gender | ClassID | TeacherID | TID | Name          | Age | Gender |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
|     1 | Shi Zhongyu |  22 | M      |       2 |         3 |   3 | Miejue Shitai |  77 | F      |
|     4 | Ding Dian   |  32 | M      |       4 |         4 |   4 | Lin Chaoying  |  93 | F      |
|     5 | Yu Yutong   |  26 | M      |       3 |         1 |   1 | Song Jiang    |  45 | M      |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+

5、左/右外连接

MariaDB [hellodb]> select * from students s inner join teachers t on  s.teacherid=t.tid;
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name        | Age | Gender | ClassID | TeacherID | TID | Name          | Age | Gender |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
|     1 | Shi Zhongyu |  22 | M      |       2 |         3 |   3 | Miejue Shitai |  77 | F      |
|     4 | Ding Dian   |  32 | M      |       4 |         4 |   4 | Lin Chaoying  |  93 | F      |
|     5 | Yu Yutong   |  26 | M      |       3 |         1 |   1 | Song Jiang    |  45 | M      |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
3 rows in set (0.001 sec)

MariaDB [hellodb]> select * from students s left outer join teachers t on  s.teacherid=t.tid;
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
| StuID | Name          | Age | Gender | ClassID | TeacherID | TID  | Name          | Age  | Gender |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
|     1 | Shi Zhongyu   |  22 | M      |       2 |         3 |    3 | Miejue Shitai |   77 | F      |
|     2 | Shi Potian    |  22 | M      |       1 |         7 | NULL | NULL          | NULL | NULL   |
|     3 | Xie Yanke     |  53 | M      |       2 |        16 | NULL | NULL          | NULL | NULL   |
|     4 | Ding Dian     |  32 | M      |       4 |         4 |    4 | Lin Chaoying  |   93 | F      |
|     5 | Yu Yutong     |  26 | M      |       3 |         1 |    1 | Song Jiang    |   45 | M      |
|     6 | Shi Qing      |  46 | M      |       5 |      NULL | NULL | NULL          | NULL | NULL   |
|     7 | Xi Ren        |  19 | F      |       3 |      NULL | NULL | NULL          | NULL | NULL   |
|     8 | Lin Daiyu     |  17 | F      |       7 |      NULL | NULL | NULL          | NULL | NULL   |
|     9 | Ren Yingying  |  20 | F      |       6 |      NULL | NULL | NULL          | NULL | NULL   |
|    10 | Yue Lingshan  |  19 | F      |       3 |      NULL | NULL | NULL          | NULL | NULL   |
|    11 | Yuan Chengzhi |  23 | M      |       6 |      NULL | NULL | NULL          | NULL | NULL   |
|    12 | Wen Qingqing  |  19 | F      |       1 |      NULL | NULL | NULL          | NULL | NULL   |
|    13 | Tian Boguang  |  33 | M      |       2 |      NULL | NULL | NULL          | NULL | NULL   |
|    14 | Lu Wushuang   |  17 | F      |       3 |      NULL | NULL | NULL          | NULL | NULL   |
|    15 | Duan Yu       |  19 | M      |       4 |      NULL | NULL | NULL          | NULL | NULL   |
|    16 | Xu Zhu        |  21 | M      |       1 |      NULL | NULL | NULL          | NULL | NULL   |
|    17 | Lin Chong     |  25 | M      |       4 |      NULL | NULL | NULL          | NULL | NULL   |
|    18 | Hua Rong      |  23 | M      |       7 |      NULL | NULL | NULL          | NULL | NULL   |
|    19 | Xue Baochai   |  18 | F      |       6 |      NULL | NULL | NULL          | NULL | NULL   |
|    20 | Diao Chan     |  19 | F      |       7 |      NULL | NULL | NULL          | NULL | NULL   |
|    21 | Huang Yueying |  22 | F      |       6 |      NULL | NULL | NULL          | NULL | NULL   |
|    22 | Xiao Qiao     |  20 | F      |       1 |      NULL | NULL | NULL          | NULL | NULL   |
|    23 | Ma Chao       |  23 | M      |       4 |      NULL | NULL | NULL          | NULL | NULL   |
|    24 | Xu Xian       |  27 | M      |    NULL |      NULL | NULL | NULL          | NULL | NULL   |
|    25 | Sun Dasheng   | 100 | M      |    NULL |      NULL | NULL | NULL          | NULL | NULL   |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
25 rows in set (0.001 sec)

MariaDB [hellodb]> select * from students s right outer join teachers t on  s.teacherid=t.tid;
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name        | Age  | Gender | ClassID | TeacherID | TID | Name          | Age | Gender |
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
|     1 | Shi Zhongyu |   22 | M      |       2 |         3 |   3 | Miejue Shitai |  77 | F      |
|     4 | Ding Dian   |   32 | M      |       4 |         4 |   4 | Lin Chaoying  |  93 | F      |
|     5 | Yu Yutong   |   26 | M      |       3 |         1 |   1 | Song Jiang    |  45 | M      |
|  NULL | NULL        | NULL | NULL   |    NULL |      NULL |   2 | Zhang Sanfeng |  94 | M      |
+-------+-------------+------+--------+---------+-----------+-----+---------------+-----+--------+
4 rows in set (0.001 sec)

6、自连接

#自连接
MariaDB [hellodb]> select * from emp;
+------+----------+----------+
| id   | name     | leaderid |
+------+----------+----------+
|    1 | mage     |     NULL |
|    2 | zhangsir |        1 |
|    3 | wang     |        2 |
|    4 | zhang   |        3 |
+------+----------+----------+
4 rows in set (0.00 sec)
MariaDB [hellodb]> select   e.name,l.name   from emp as e inner join emp as l on
e.leaderid=l.id;
+----------+----------+
| name     | name     |
+----------+----------+
| zhangsir | mage     |
| wang     | zhangsir |
| zhang   | wang     |
+----------+----------+
3 rows in set (0.00 sec)