多表查询：
嵌套查询
where关联查询
join连接查询（左联，右联，内联）
嵌套查询 (不推荐)
在SQL语言中，一个SELECT-FROM-WHERE语句称为一个查询块。将一个查询块嵌套在另一个查询块的WHERE子句或HAVING短语的条件的查询称为嵌套查询。
先处理内查询由内向外查询
mysql> select max(age) from stu;
+----------+
| max(age) |
+----------+
| 29 |
+----------+
1 row in set (0.00 sec)
mysql> select * from stu where age=29;
+----+---------+------+-----+---------+
| id | name | age | sex | classid |
+----+---------+------+-----+---------+
| 9 | xiaoli | 29 | w | 2 |
| 14 | zhangle | 29 | m | 5 |
+----+---------+------+-----+---------+
2 rows in set (0.01 sec)
-- 查询年龄最大的所有学生信息
mysql> select * from stu where age=(select max(age) from stu);
+----+---------+------+-----+---------+
| id | name | age | sex | classid |
+----+---------+------+-----+---------+
| 9 | xiaoli | 29 | w | 2 |
| 14 | zhangle | 29 | m | 5 |
+----+---------+------+-----+---------+
2 rows in set (0.01 sec)
-- 查询python02期的所有学生信息
mysql> select * from stu where classid=(select id from classes where name='python02');
mysql> select * from stu where classid in(select id from classes where name='python02');
+----+---------+------+-----+---------+
| id | name | age | sex | classid |
+----+---------+------+-----+---------+
| 2 | lisi | 25 | m | 2 |
| 6 | uu02 | 25 | m | 2 |
| 7 | uu03 | 28 | w | 2 |
| 9 | xiaoli | 29 | w | 2 |
| 11 | xiaoyan | 22 | m | 2 |
| 13 | wangwen | 27 | w | 2 |
+----+---------+------+-----+---------+
6 rows in set (0.00 sec)
where关联查询
-- 查询所有学生信息，并跨表显示对应的班级名称信息
mysql> select s.*,c.name cname from stu s,classes c where s.classid=c.id;
+----+-----------+------+-----+---------+----------+
| id | name | age | sex | classid | cname |
+----+-----------+------+-----+---------+----------+
| 1 | zhangsan | 20 | w | 1 | python01 |
| 2 | lisi | 25 | m | 2 | python02 |
| 3 | wangwu | 22 | w | 5 | python04 |
| 4 | zhaoliu | 21 | m | 4 | python03 |
| 5 | uu01 | 27 | w | 1 | python01 |
| 6 | uu02 | 25 | m | 2 | python02 |
| 7 | uu03 | 28 | w | 2 | python02 |
| 8 | uu05 | 22 | m | 4 | python03 |
| 9 | xiaoli | 29 | w | 2 | python02 |
| 10 | xiaozhang | 19 | w | 1 | python01 |
| 11 | xiaoyan | 22 | m | 2 | python02 |
| 12 | xiaoxin | 28 | w | 4 | python03 |
| 13 | wangwen | 27 | w | 2 | python02 |
| 14 | zhangle | 29 | m | 5 | python04 |
+----+-----------+------+-----+---------+----------+
14 rows in set (0.01 sec)
-- 统计每个班级的人数
mysql> select c.name,count(s.id) from classes c,stu s where c.id=s.classid group by c.id;
+----------+-------------+
| name | count(s.id) |
+----------+-------------+
| python01 | 3 |
| python02 | 6 |
| python03 | 3 |
| python04 | 2 |
+----------+-------------+
4 rows in set (0.00 sec)
mysql> select c.name,count(s.id) from classes c,stu s where c.id=s.classid group by c.id order by count(s.id) desc;
+----------+-------------+
| name | count(s.id) |
+----------+-------------+
| python02 | 6 |
| python01 | 3 |
| python03 | 3 |
| python04 | 2 |
+----------+-------------+
4 rows in set (0.00 sec)
JOIN连接查询（左联，右联，内联）
JOIN 按照功能大致分为如下三类：
- INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录。
- LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
- RIGHT JOIN（右连接）：与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。
-- 查询新闻信息，并补齐新闻类别信息
mysql> select n.id,n.title,t.name from news n,ntype t where n.ntid=t.id;
mysql> select n.id,n.title,t.name from news n inner join ntype t on n.ntid=t.id;
+----+-------------------------------------------------------------+--------------+
| id | title | name |
+----+-------------------------------------------------------------+--------------+
| 1 | 骑共享单车违法 将被禁用或冻结账户 | 娱乐新闻 |
| 2 | 武警特战排爆手 | 军事新闻 |
| 3 | 国外武装直升机型号发展与作战能力分析 | 军事新闻 |
| 4 | 俄战机过去一周在边境拦截外国侦察机11次 | 军事新闻 |
| 5 | 中国空军战机进行远洋训练。 | 军事新闻 |
| 6 | 中国驻韩使馆举行中韩建交25周年纪念招待会 | 国际新闻 |
+----+-------------------------------------------------------------+--------------+
6 rows in set (0.00 sec)
-- 同上，但采用的是左联查询left join
mysql> select n.id,n.title,t.name from news n left join ntype t on n.ntid=t.id;
+----+-------------------------------------------------------------+--------------+
| id | title | name |
+----+-------------------------------------------------------------+--------------+
| 1 | 骑共享单车违法 将被禁用或冻结账户 | 娱乐新闻 |
| 2 | 武警特战排爆手 | 军事新闻 |
| 3 | 国外武装直升机型号发展与作战能力分析 | 军事新闻 |
| 4 | 俄战机过去一周在边境拦截外国侦察机11次 | 军事新闻 |
| 5 | 中国空军战机进行远洋训练。 | 军事新闻 |
| 6 | 中国驻韩使馆举行中韩建交25周年纪念招待会 | 国际新闻 |
+----+-------------------------------------------------------------+--------------+
6 rows in set (0.01 sec)
-- 统计每个新闻类别下的新闻数量，采用where关联统计
mysql> select t.name,count(n.id) from ntype t,news n where t.id=n.ntid group by t.id;
+--------------+-------------+
| name | count(n.id) |
+--------------+-------------+
| 娱乐新闻 | 1 |
| 国际新闻 | 1 |
| 军事新闻 | 4 |
+--------------+-------------+
3 rows in set (0.01 sec)
-- 统计每个新闻类别下的新闻数量，采用左联统计
mysql> select t.name,count(n.id) from ntype t left join news n on t.id=n.ntid group by t.id;
+--------------+-------------+
| name | count(n.id) |
+--------------+-------------+
| 娱乐新闻 | 1 |
| 体育新闻 | 0 |
| 国际新闻 | 1 |
| 军事新闻 | 4 |
+--------------+-------------+
4 rows in set (0.01 sec)