Mysql SQL-LEFT联接，但我希望COUNT（*）只对联接内部的结果进行计数_Mysql_Sql

Mysql SQL-LEFT联接，但我希望COUNT（*）只对联接内部的结果进行计数

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Mysql SQL-LEFT联接，但我希望COUNT（*）只对联接内部的结果进行计数,mysql,sql,Mysql,Sql,我想显示每个客户的购买数量。如果他们购买了0，我想显示0 所需输出： ------------------------------------- | customer_name | number_of_purchases | ------------------------------------- | Marg | 0 | | Ben | 1 | | Phil |

我想显示每个客户的购买数量。如果他们购买了0，我想显示0

所需输出：

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          0          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          0          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          1          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          1          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Ben        |          1          |
|    Phil       |          4          |
 -------------------------------------

create table customer(customer_id integer, customer_name varchar(20));

create table purchaser(purchaser_id varchar(20), customer_id integer, description varchar(20));


insert into customer values(1, 'Marg');
insert into customer values(2, 'Ben');
insert into customer values(3, 'Phil');
insert into customer values(4, 'Steve');

insert into purchaser values(1, 2, '500 Reams');
insert into purchaser values(2, 3, '6 toners');
insert into purchaser values(3, 3, '20 Staplers');
insert into purchaser values(4, 3, '20 Staplers');
insert into purchaser values(5, 3, '20 Staplers');

SELECT c.customer_id, c.customer_name, COUNT(p.purchaser_id) AS number_of_purchaes 
FROM customer c
LEFT JOIN purchaser p ON c.customer_id = p.customer_id 
GROUP BY c.customer_id;

客户表格：

 -----------------------------
| customer_id | customer_name |
 -----------------------------
|      1      |      Marg     |
|      2      |      Ben      |
|      3      |      Phil     |
|      4      |      Steve    |
 -----------------------------

 --------------------------------------------------
| purchase_id | customer_id | purchase_description |
 --------------------------------------------------
|      1      |       2     |     500 Reams        |
|      2      |       3     |     6 Toners         |
|      3      |       3     |     20 Staplers      |
|      4      |       3     |     2 Copiers        |
|      5      |       3     |     9 Name Plaques   |
 --------------------------------------------------

采购表格：

 -----------------------------
| customer_id | customer_name |
 -----------------------------
|      1      |      Marg     |
|      2      |      Ben      |
|      3      |      Phil     |
|      4      |      Steve    |
 -----------------------------

 --------------------------------------------------
| purchase_id | customer_id | purchase_description |
 --------------------------------------------------
|      1      |       2     |     500 Reams        |
|      2      |       3     |     6 Toners         |
|      3      |       3     |     20 Staplers      |
|      4      |       3     |     2 Copiers        |
|      5      |       3     |     9 Name Plaques   |
 --------------------------------------------------

我目前的查询如下：

SELECT customer_name, COUNT(*) AS number_of_purchaes 
FROM customer 
LEFT JOIN purchases ON customer.customer_id = purchases.customer_id 
GROUP BY customer.customer_id

但是，由于它是一个

左连接

，因此查询会为没有购买的客户生成行，这使他们成为

计数（*）的一部分。换句话说，购买了0件商品的客户显示为购买了1件商品，如下所示：
左连接输出：
 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          0          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          0          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          1          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          1          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Ben        |          1          |
|    Phil       |          4          |
 -------------------------------------

create table customer(customer_id integer, customer_name varchar(20));

create table purchaser(purchaser_id varchar(20), customer_id integer, description varchar(20));


insert into customer values(1, 'Marg');
insert into customer values(2, 'Ben');
insert into customer values(3, 'Phil');
insert into customer values(4, 'Steve');

insert into purchaser values(1, 2, '500 Reams');
insert into purchaser values(2, 3, '6 toners');
insert into purchaser values(3, 3, '20 Staplers');
insert into purchaser values(4, 3, '20 Staplers');
insert into purchaser values(5, 3, '20 Staplers');

SELECT c.customer_id, c.customer_name, COUNT(p.purchaser_id) AS number_of_purchaes 
FROM customer c
LEFT JOIN purchaser p ON c.customer_id = p.customer_id 
GROUP BY c.customer_id;

我还尝试了内部连接
，但这会导致0次购买的客户根本不显示：
内部联接输出：
 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          0          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          0          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          1          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          1          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Ben        |          1          |
|    Phil       |          4          |
 -------------------------------------

create table customer(customer_id integer, customer_name varchar(20));

create table purchaser(purchaser_id varchar(20), customer_id integer, description varchar(20));


insert into customer values(1, 'Marg');
insert into customer values(2, 'Ben');
insert into customer values(3, 'Phil');
insert into customer values(4, 'Steve');

insert into purchaser values(1, 2, '500 Reams');
insert into purchaser values(2, 3, '6 toners');
insert into purchaser values(3, 3, '20 Staplers');
insert into purchaser values(4, 3, '20 Staplers');
insert into purchaser values(5, 3, '20 Staplers');

SELECT c.customer_id, c.customer_name, COUNT(p.purchaser_id) AS number_of_purchaes 
FROM customer c
LEFT JOIN purchaser p ON c.customer_id = p.customer_id 
GROUP BY c.customer_id;

我如何才能实现我的期望输出，其中显示了购买量为0的客户？
而不是计数（*）
使用计数（购买id）

COUNT（*）
对行进行计数。要计算匹配项，请从第二个表中进行计数，如下所示：
select customer.customer_name , a.number_of_purchases from (
SELECT customer_id, COUNT(purchases.purchase_id) AS number_of_purchaes 
FROM customer 
LEFT JOIN purchases ON customer.customer_id = purchases.customer_id 
GROUP BY customer.customer_id) as a 
inner join customer on customer.customer_id=a.customer_id;

换句话说，LEFT JOIN
在没有匹配项时返回一行。该行对于purchases
表中的所有列都有一个NULL
值。
您可以这样尝试：
样本数据：
 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          0          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          0          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Marg       |          1          |
|    Ben        |          1          |
|    Phil       |          4          |
|    Steve      |          1          |
 -------------------------------------

 -------------------------------------
| customer_name | number_of_purchases |
 -------------------------------------
|    Ben        |          1          |
|    Phil       |          4          |
 -------------------------------------

create table customer(customer_id integer, customer_name varchar(20));

create table purchaser(purchaser_id varchar(20), customer_id integer, description varchar(20));


insert into customer values(1, 'Marg');
insert into customer values(2, 'Ben');
insert into customer values(3, 'Phil');
insert into customer values(4, 'Steve');

insert into purchaser values(1, 2, '500 Reams');
insert into purchaser values(2, 3, '6 toners');
insert into purchaser values(3, 3, '20 Staplers');
insert into purchaser values(4, 3, '20 Staplers');
insert into purchaser values(5, 3, '20 Staplers');

SELECT c.customer_id, c.customer_name, COUNT(p.purchaser_id) AS number_of_purchaes 
FROM customer c
LEFT JOIN purchaser p ON c.customer_id = p.customer_id 
GROUP BY c.customer_id;

SQL fiddle:
而不是count（*）使用count（purchases.customer\u id）
COUNT（*）返回组中的项目数。这包括空值和重复项
COUNT（ALL expression）为组中的每一行计算表达式，并返回非空值的数目
CREATE table customer(customer_id integer , customer_name varchar(20));

create table purchases(purchase_id integer , customer_id integer , purchase_description varchar(30));

INSERT INTO customer ( customer_id, customer_name )
VALUES ( 1, 'Marg' )
     , ( 2, 'Ben' )
     , ( 3, 'Phil' )
     , ( 4, 'Steve' );

INSERT INTO purchases ( purchase_id, customer_id, purchase_description )
VALUES ( 1, 2, '500 Reams' )
     , ( 2, 3, '6 toners' )
     , ( 3, 3, '20 Staplers' )
     , ( 4, 3, '2 Copiers' )
     , ( 5, 3, '9 Name Plaques' );


 SELECT  c.customer_name
      , COUNT(p.purchase_id) AS number_of_purchases
FROM    customer c
        LEFT JOIN purchases p
            ON c.customer_id = p.customer_id
GROUP BY c.customer_name

SUM（ISNULL（purchases.customer\u id））作为采购数量
我已经添加了答案。检查是否添加了代码和sql FIDLE。customer\u name
不属于分组依据
。除非它不对，否则不会起作用customer_name
位于select
列表中，但不属于任何aggregated函数
或groupby
子句的一部分。在MySQL中，您不必按所有列进行分组。你可以根据你想要的任何列进行分组。在mysql
中可能没有那么严格Oracle
和SQL Server
不允许这样做。5.7+将允许这样做，即使在set SQL\u mode=ONLY\u FULL\u GROUP\u BY
中，只要客户id是PK或唯一的非空辅助密钥。当group by字段是唯一的时，同一表中的其他行不能是无效的聚合，因为它们只能有一个值。请在group by子句中包含customer_id，以避免假设名称是唯一的。