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# MYSQL查询相关

演示的表和插入语句

```mysql
# 建立表
mysql> CREATE TABLE fruits
-> (
-> f_id char(10) NOT NULL, # 水果id
-> s_id INT NOT NULL, # 供应商id
-> f_name char(255) NOT NULL, # 水果名字
-> f_price decimal(8,2) NOT NULL, # 水果价格
-> PRIMARY KEY(f_id)
-> );
Query OK, 0 rows affected (0.06 sec)

# 插入
mysql> INSERT INTO fruits (f_id, s_id, f_name, f_price)
-> VALUES('a1', 101,'apple',5.2),
-> ('b1',101,'blackberry', 10.2),
-> ('bs1',102,'orange', 11.2),
-> ('bs2',105,'melon',8.2),
-> ('t1',102,'banana', 10.3),
-> ('t2',102,'grape', 5.3),
-> ('o2',103,'coconut', 9.2),
-> ('c0',101,'cherry', 3.2),
-> ('a2',103, 'apricot',2.2),
-> ('l2',104,'lemon', 6.4),
-> ('b2',104,'berry', 7.6),
-> ('m1',106,'mango', 15.6),
-> ('m2',105,'xbabay', 2.6),
-> ('t4',107,'xbababa', 3.6),
-> ('m3',105,'xxtt', 11.6),
-> ('b5',107,'xxxx', 3.6);
Query OK, 16 rows affected (0.01 sec)
Records: 16 Duplicates: 0 Warnings: 0

# 查询所有
mysql> select * from fruits;
+------+------+------------+---------+
| f_id | s_id | f_name | f_price |
+------+------+------------+---------+
| a1 | 101 | apple | 5.20 |
| a2 | 103 | apricot | 2.20 |
| b1 | 101 | blackberry | 10.20 |
| b2 | 104 | berry | 7.60 |
| b5 | 107 | xxxx | 3.60 |
| bs1 | 102 | orange | 11.20 |
| bs2 | 105 | melon | 8.20 |
| c0 | 101 | cherry | 3.20 |
| l2 | 104 | lemon | 6.40 |
| m1 | 106 | mango | 15.60 |
| m2 | 105 | xbabay | 2.60 |
| m3 | 105 | xxtt | 11.60 |
| o2 | 103 | coconut | 9.20 |
| t1 | 102 | banana | 10.30 |
| t2 | 102 | grape | 5.30 |
| t4 | 107 | xbababa | 3.60 |
+------+------+------------+---------+
16 rows in set (0.00 sec)

mysql>
```

## 一、分组查询

### 1、基本分组操作

对数据按照某个或多个字段进行分组，MYSQL中使用`group by `关键字对数据进行分组。基本形式为:

```mysql
group by 字段 having <条件表达式>
```

`group by `关键字通常和集合函数一起使用，例如`MAX()、MIN()、COUNT()、SUM()、AVG()`。

例如: 根据`si_d`对`fruits`表中的数据进行分组:

```mysql
mysql> select s_id, count(*) as Total from fruits group by s_id;
+------+-------+
| s_id | Total |
+------+-------+
| 101 | 3 |
| 102 | 3 |
| 103 | 2 |
| 104 | 2 |
| 105 | 3 |
| 106 | 1 |
| 107 | 2 |
+------+-------+
7 rows in set (0.00 sec)

```

`s_id`表示供应商的`ID`。`Total`字段使用`COUNT()`函数计算得出。

`GROUP BY`字句按照`s_id`先**排序**并对数据进行分组。

如果要查询所有种类的名称，可以使用`GROUP_CONCAT()`函数。

```java
mysql> select s_id, GROUP_CONCAT(f_name) AS Names FROM fruits GROUP BY s_id;
+------+-------------------------+
| s_id | Names |
+------+-------------------------+
| 101 | apple,blackberry,cherry |
| 102 | orange,banana,grape |
| 103 | apricot,coconut |
| 104 | berry,lemon |
| 105 | melon,xbabay,xxtt |
| 106 | mango |
| 107 | xxxx,xbababa |
+------+-------------------------+
7 rows in set (0.00 sec)

mysql>

```

使用`having`过滤分组：

**GROUP BY**可以和`HAVING`一起限定显示记录所需要满足的条件。只有满足条件的分组才会被显示。

例如，查询水果种类大于`>1`的分组信息。

```java
mysql> select s_id, GROUP_CONCAT(f_name) as Names from fruits GROUP BY s_id HAVING count(f_name) > 1;
+------+-------------------------+
| s_id | Names |
+------+-------------------------+
| 101 | apple,blackberry,cherry |
| 102 | orange,banana,grape |
| 103 | apricot,coconut |
| 104 | berry,lemon |
| 105 | melon,xbabay,xxtt |
| 107 | xxxx,xbababa |
+------+-------------------------+
6 rows in set (0.01 sec)

```

可以看到由于`s_id = 106`的**供应商**的水果种类只有一种。所以不在结果中。

### 2、 `GROUP BY` **关键字和`WHERE`关键字都是用来过滤数据，有什么区别呢**？

答:

* `HAVING`在分组之后进行过滤。
* `WHERE`在分组之前过滤。

看例子:

①都where和having都可以使用的场景:

```java
mysql> select f_id, f_price from fruits where f_price > 10.00;
+------+---------+
| f_id | f_price |
+------+---------+
| b1 | 10.20 |
| bs1 | 11.20 |
| m1 | 15.60 |
| m3 | 11.60 |
| t1 | 10.30 |
+------+---------+
5 rows in set (0.00 sec)

mysql> select f_id, f_price from fruits having f_price > 10.00;
+------+---------+
| f_id | f_price |
+------+---------+
| b1 | 10.20 |
| bs1 | 11.20 |
| m1 | 15.60 |
| m3 | 11.60 |
| t1 | 10.30 |
+------+---------+
5 rows in set (0.00 sec)

```

原因: `f_price`作为条件也出现在了查询字段中。

②只可以使用where，不可以使用having的情况：

```java
mysql> select f_id, f_name from fruits where f_price > 10.00;
+------+------------+
| f_id | f_name |
+------+------------+
| b1 | blackberry |
| bs1 | orange |
| m1 | mango |
| m3 | xxtt |
| t1 | banana |
+------+------------+
5 rows in set (0.01 sec)

mysql> select f_id, f_name from fruits having f_price > 10.00;
ERROR 1054 (42S22): Unknown column 'f_price' in 'having clause'
mysql>

```

原因: `f_price`没有在查询语句中出现，所以不能用`having`。

③只可以使用having，不可以使用where的情况：

查询供应商供应的种类`>2`的情况。

```java
mysql> select s_id, GROUP_CONCAT(f_name), COUNT(f_name) as al from fruits GROUP BY s_id HAVING al > 2;
+------+-------------------------+----+
| s_id | GROUP_CONCAT(f_name) | al |
+------+-------------------------+----+
| 101 | apple,blackberry,cherry | 3 |
| 102 | orange,banana,grape | 3 |
| 105 | melon,xbabay,xxtt | 3 |
+------+-------------------------+----+
3 rows in set (0.00 sec)

mysql> select s_id, GROUP_CONCAT(f_name), COUNT(f_name) as al from fruits where al > 2 GROUP BY s_id;
ERROR 1054 (42S22): Unknown column 'al' in 'where clause'
mysql>

```

原因: 因为`where`是在分组之前过滤，所以那个时候还没有`al`这个变量。

### 3、多字段分组，以及和order by一起使用

先按照`s_id`分组，然后按照`f_name`分组。这里在查询的时候，可能会报错。解决方案:

<https://blog.csdn.net/qq_31365675/article/details/81010650>

查询代码:

```mysql
mysql> select * from fruits GROUP BY s_id,f_name;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'lmysql.fruits.f_id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
# 解决错误的语句
mysql> SET sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));
Query OK, 0 rows affected (0.01 sec)
mysql> select * from fruits GROUP BY s_id,f_name;
+------+------+------------+---------+
| f_id | s_id | f_name | f_price |
+------+------+------------+---------+
| a1 | 101 | apple | 5.20 |
| b1 | 101 | blackberry | 10.20 |
| c0 | 101 | cherry | 3.20 |
| t1 | 102 | banana | 10.30 |
| t2 | 102 | grape | 5.30 |
| bs1 | 102 | orange | 11.20 |
| a2 | 103 | apricot | 2.20 |
| o2 | 103 | coconut | 9.20 |
| b2 | 104 | berry | 7.60 |
| l2 | 104 | lemon | 6.40 |
| bs2 | 105 | melon | 8.20 |
| m2 | 105 | xbabay | 2.60 |
| m3 | 105 | xxtt | 11.60 |
| m1 | 106 | mango | 15.60 |
| t4 | 107 | xbababa | 3.60 |
| b5 | 107 | xxxx | 3.60 |
+------+------+------------+---------+
16 rows in set (0.00 sec)

```

使用`group by`和`order by `结合:

要对求出来的`SUM(quantity * item_price) AS orderTotal`进行排序

```java
mysql> INSERT INTO orderitems(o_num, o_item, f_id, quantity, item_price)
-> VALUES(30001, 1, 'a1', 10, 5.2),
-> (30001, 2, 'b2', 3, 7.6),
-> (30001, 3, 'bs1', 5, 11.2),
-> (30001, 4, 'bs2', 15, 9.2),
-> (30002, 1, 'b3', 2, 20.0),
-> (30003, 1, 'c0', 100, 10),
-> (30004, 1, 'o2', 50, 2.50),
-> (30005, 1, 'c0', 5, 10),
-> (30005, 2, 'b1', 10, 8.99),
-> (30005, 3, 'a2', 10, 2.2),
-> (30005, 4, 'm1', 5, 14.99);
Query OK, 11 rows affected (0.01 sec)
Records: 11 Duplicates: 0 Warnings: 0

mysql> SELECT o_num, SUM(quantity * item_price) AS orderTotal
-> FROM orderitems
-> GROUP BY o_num
-> HAVING SUM(quantity*item_price) >= 100;
+-------+------------+
| o_num | orderTotal |
+-------+------------+
| 30001 | 268.80 |
| 30003 | 1000.00 |
| 30004 | 125.00 |
| 30005 | 236.85 |
+-------+------------+
4 rows in set (0.01 sec)

mysql> SELECT o_num, SUM(quantity * item_price) AS orderTotal
-> FROM orderitems
-> GROUP BY o_num
-> HAVING SUM(quantity*item_price) >= 100
-> ORDER BY orderTotal;
+-------+------------+
| o_num | orderTotal |
+-------+------------+
| 30004 | 125.00 |
| 30005 | 236.85 |
| 30001 | 268.80 |
| 30003 | 1000.00 |
+-------+------------+
4 rows in set (0.00 sec)

```

## 二、聚合函数

### 1、count

使用`count(*)`和`count(列)`的区别: `count(*)`是统计总的行数，而`count(列)`是统计这一列不为空的数目。

```java
mysql> select * from customers;
+-------+----------+---------------------+---------+--------+-----------+-------------------+
| c_id | c_name | c_address | c_city | c_zip | c_contact | c_email |
+-------+----------+---------------------+---------+--------+-----------+-------------------+
| 10001 | RedHook | 200 Street | Tianjin | 300000 | LiMing | LMing@163.com |
| 10002 | Stars | 333 Fromage Lane | Dalian | 116000 | Zhangbo | Jerry@hotmail.com |
| 10003 | Netbhood | 1 Sunny Place | Qingdao | 266000 | LuoCong | NULL |
| 10004 | JOTO | 829 Riverside Drive | Haikou | 570000 | YangShan | sam@hotmail.com |
+-------+----------+---------------------+---------+--------+-----------+-------------------+
4 rows in set (0.00 sec)

mysql> select count(c_email) from customers;
+----------------+
| count(c_email) |
+----------------+
| 3 |
+----------------+
1 row in set (0.00 sec)

mysql> select count(*) from customers;
+----------+
| count(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)

mysql>

```



### 2、sum

```java
mysql> select * from orderitems;
+-------+--------+------+----------+------------+
| o_num | o_item | f_id | quantity | item_price |
+-------+--------+------+----------+------------+
| 30001 | 1 | a1 | 10 | 5.20 |
| 30001 | 2 | b2 | 3 | 7.60 |
| 30001 | 3 | bs1 | 5 | 11.20 |
| 30001 | 4 | bs2 | 15 | 9.20 |
| 30002 | 1 | b3 | 2 | 20.00 |
| 30003 | 1 | c0 | 100 | 10.00 |
| 30004 | 1 | o2 | 50 | 2.50 |
| 30005 | 1 | c0 | 5 | 10.00 |
| 30005 | 2 | b1 | 10 | 8.99 |
| 30005 | 3 | a2 | 10 | 2.20 |
| 30005 | 4 | m1 | 5 | 14.99 |
+-------+--------+------+----------+------------+
11 rows in set (0.00 sec)

mysql> select o_num, count(f_id) from orderitems group by o_num;
+-------+-------------+
| o_num | count(f_id) |
+-------+-------------+
| 30001 | 4 |
| 30002 | 1 |
| 30003 | 1 |
| 30004 | 1 |
| 30005 | 4 |
+-------+-------------+
5 rows in set (0.00 sec)

```

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