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blog/php-mysql/sql-senior-select.md
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......@@ -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
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https://www.bilibili.com/video/BV1Vx411g7uJ?p=39&spm_id_from=pageDriver
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