fix

blog/php-mysql/sql-senior-select.md

@@ -5,6 +5,8 @@
 ```sql
 select 选项 字段列表 
 from 数据源 
+
+-- 5子句
 where 条件 
 group by 分组 
 having 条件 
@@ -12,6 +14,8 @@ order by 排序
 limit 限制;
 ```
 
+
+
 ## 1、select选项
 
 处理查询到的结果
@@ -155,7 +159,7 @@ mysql> select * from (select * from my_student) as t1;
 
 根据指定的字段，将数据进行分组，分组的目的是为了统计
 
-## 5.1、分组统计
+### 5.1、分组统计
 
 ```sql
 group by 字段名
@@ -182,7 +186,7 @@ mysql> select  class_id  from my_student group by class_id;
 +----------+
 ```
 
-## 5.2、统计函数（聚合函数）
+### 5.2、聚合函数
 
 - count() 统计数量。如果是字段，不统计null字段
 - avg 平均值
@@ -221,20 +225,285 @@ mysql> select class_id, group_concat(name), count(*), max(age), min(age), avg(ag
 +----------+--------------------+----------+----------+----------+----------+
 ```
 
+### 5.3、多分组
+
+对已分组的数据进行再次分组
+
+基本语法
+```sql
+-- 按照字段1进行分组，将结果再按照字段2进行分组
+group by 字段1, 字段2;
+```
+
+```sql
+mysql> select * from my_student;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  4 | 张飞   |        2 |   21 |      1 |
+|  5 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+
+mysql> select class_id, gender, count(*), group_concat(name) from my_student group by class_id, gender;
++----------+--------+----------+--------------------+
+| class_id | gender | count(*) | group_concat(name) |
++----------+--------+----------+--------------------+
+|        1 |      1 |        1 | 李四               |
+|        1 |      2 |        2 | 刘备,关羽          |
+|        2 |      1 |        1 | 张飞               |
+|        2 |      2 |        1 | 王五               |
++----------+--------+----------+--------------------+
+```
 
-多分组
-分组排序
-回溯统计
+### 5.4、分组排序
 
+按照分组字段排序，默认升序
+```sql
+-- 班级升序，性别降序 
+-- mysql8.012之后，不支持group by 排序，需要使用order by排序
+select class_id, gender, count(*), group_concat(name) 
+from my_student 
+group by class_id, gender 
+order by class_id asc, gender desc;
++----------+--------+----------+--------------------+
+| class_id | gender | count(*) | group_concat(name) |
++----------+--------+----------+--------------------+
+|        1 |      2 |        2 | 刘备,关羽          |
+|        1 |      1 |        1 | 李四               |
+|        2 |      2 |        1 | 王五               |
+|        2 |      1 |        1 | 张飞               |
++----------+--------+----------+--------------------+
+```
 
+### 5.5、回溯排序
 
-having
+统计过程中层层上报
 
-order
 
-limit
+```sql
+group by 字段 with rollup;
+```
+```sql
+-- 分组
+mysql> select class_id, count(*) from my_student group by class_id;
++----------+----------+
+| class_id | count(*) |
++----------+----------+
+|        1 |        3 |
+|        2 |        2 |
++----------+----------+
+
+-- 分组回溯
+mysql> select class_id, count(*) from my_student group by class_id with rollup;
++----------+----------+
+| class_id | count(*) |
++----------+----------+
+|        1 |        3 |
+|        2 |        2 |
+|     NULL |        5 |
++----------+----------+
+
+-- 多分组
+mysql> select class_id, gender, count(*) from my_student group by class_id, gender;
++----------+--------+----------+
+| class_id | gender | count(*) |
++----------+--------+----------+
+|        1 |      2 |        2 |
+|        1 |      1 |        1 |
+|        2 |      2 |        1 |
+|        2 |      1 |        1 |
++----------+--------+----------+
+
+-- 多分组回溯
+mysql> select class_id, gender, count(*) from my_student group by class_id, gender with rollup;
++----------+--------+----------+
+| class_id | gender | count(*) |
++----------+--------+----------+
+|        1 |      1 |        1 |
+|        1 |      2 |        2 |
+|        1 |   NULL |        3 |
+|        2 |      1 |        1 |
+|        2 |      2 |        1 |
+|        2 |   NULL |        2 |
+|     NULL |   NULL |        5 |
++----------+--------+----------+
+```
+
+## 6、having条件
+
+和where一样，用来进行数据条件筛选
+
+区别：
+- where是从表中取数据，where将数据从磁盘拿到内存，where之后的操作都是内存操作
+- having聚合之后的数据中取数据
+
+用在group by分组之后，可以针对分组数据进行统计筛选
+
+```sql
+mysql> select * from my_student;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+
+
+-- 查询班级人数大于等于3以上的班级
+mysql> select class_id, count(*) as total 
+from my_student 
+group by class_id 
+having total >= 3;
++----------+-------+
+| class_id | total |
++----------+-------+
+|        1 |     3 |
++----------+-------+
+```
+
+## 7、order by排序
+
+### 7.1、单字段排序
+
+基本语法
+```sql
+-- 默认asc升序，desc降序
+order by 字段 [asc|desc] 
+```
+
+```sql
+-- 按照年龄降序排序
+mysql> select * from my_student order by age asc;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+```
 
-聚合函数
+### 7.2、多字段排序
 
+基本语法
+```sql
+order by 字段1, 字段2... [asc|desc];
+```
+
+```sql
+-- 按照班级和年龄排序
+mysql> select * from my_student order by class_id, age desc;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  8 | 关羽   |        1 |   22 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  1 | 刘备   |        1 |   18 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
++----+--------+----------+------+--------+
+```
+
+## 8、limit 限制
+
+限制记录数数量，如果数量不够，仅返回剩余数据
+
+### 8.1、记录数限制
+
+基本语法
+```sql
+limit 数量;
+```
+
+```sql
+mysql> select * from my_student;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+
+mysql> select * from my_student limit 2;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
++----+--------+----------+------+--------+
+```
+
+### 8.2、分页
+
+获取指定区间的数据
+
+基本语法
+
+```sql
+limit 偏移量, 数量;
+
+-- 等价于
+limit 数量 offset 偏移量;
+```
+
+MySQL下标从0开始
+
+分页计算公式：
+```
+page: 页数
+size: 每页数量
+
+偏移量 = (page - 1) * size
+```
+
+```sql
+mysql> select * from my_student;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+
+-- 每页2条数据，获取第1页 (1 - 1) * 2, 2
+mysql> select * from my_student limit 0, 2;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
++----+--------+----------+------+--------+
+
+-- 每页2条数据，获取第2页 (2 - 1) * 2, 2
+mysql> select * from my_student limit 2, 2;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
++----+--------+----------+------+--------+
+
+-- 每页3条数据，获取第2页 (3 - 1) * 2, 2
+mysql> select * from my_student limit 4, 2;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+```
 
-https://www.bilibili.com/video/BV1Vx411g7uJ?p=37&spm_id_from=pageDriver
+https://www.bilibili.com/video/BV1Vx411g7uJ?p=39&spm_id_from=pageDriver
\ No newline at end of file