MySQL使用JOIN COUNT多个表

Question

我尝试使用连接从多个表中选择总数时遇到问题.结果COUNT不正确.

我有三张桌子:

Customers
id -> Primary/Autoincrement
name

Documents
id -> Primary/Autoincrement
customer_id

Documents_items
id -> Primary/Autoincrement
document_id

我想获得按客户名称分组的文件和文件项目总数.

    SELECT cust.name, 
           COUNT(doc.id), 
           COUNT(item.id)
      FROM customers AS cust
INNER JOIN documents AS doc ON doc.customer_id = cust.id
INNER JOIN documents_items AS item ON item.document_id = doc.id
  GROUP BY cust.name

问题是结果COUNT(doc.id)等于结果COUNT(item.id)不正确.

您可以在SQLFiddle中看到错误的演示示例.

输入示例:

INSERT INTO customers VALUES('John')
INSERT INTO documents VALUES(1)
INSERT INTO documents_items VALUES(1), VALUES(1)

预期产量:

Name     |    Total Docs    | Total Items
John              1               2

当前输出:

Name     |    Total Docs    | Total Items
John              2               2

Answer 1

您想要计算不同的文档ID和项ID.

    SELECT cust.name, 
           COUNT(DISTINCT doc.id), 
           COUNT(DISTINCT item.id)
      FROM customers AS cust
INNER JOIN documents AS doc ON doc.customer_id = cust.id
INNER JOIN documents_items AS item ON item.document_id = doc.id
  GROUP BY cust.name