多个内部联接 - MySQL

Question

我有两张表Task和Transaction，如下所示：

任务

交易

我想创建一个从两个表中返回信息的查询。

列：name, priority, waiting, error, done, total, status

其中：

• name = Task.name
• priority = Task.priority
• waiting = count（transaction.id）WHERE task.id = transaction.task and transaction.status = 1
• error = count（transaction.id）WHERE task.id = transaction.task and transaction.status = 2
• done = count（transaction.id）WHERE task.id = transaction.task and transaction.status = 3
• total = count（transaction.id）WHERE task.id = transaction.task
• status = task.status

我尝试使用INNER加入，但结果错误：

SELECT tk.name, tk.priority, waiting.waiting, error.error, done.done, total.total
  FROM task AS tk, transaction AS tran
  INNER JOIN (
    SELECT count(id) AS waiting
    FROM transaction
    WHERE status = 1
  ) AS waiting
  INNER JOIN (
    SELECT count(id) AS error
    FROM transaction
    WHERE status = 3
  ) AS error
  INNER JOIN (
    SELECT count(id) AS done
    FROM transaction
    WHERE status = 4
  ) AS done
  INNER JOIN (
    SELECT count(id) AS total
    FROM transaction
  ) AS total;

你能帮我创建一下这个查询吗？我得到列等待，错误，完成，总计与所有交易的计数。相反，它应该获得事务数WHERE task.id = transaction.task和transaction.status = 1,2,3。

Answer 1

    select task.name,
               task.priority ,
                 count(tw.id) as waiting ,
                 count(te.id) as error,
                 count(td.id) as done,
                 count(task.id) as total,
                 task.status
        from transaction as tr
                    JOIN task 
                        on task.id = tr.task 
        left  JOIN task as tw
                        on ( tw.id = tr.task and tr.status = 1)
        left  JOIN task as te 
                        on ( te.id = tr.task and tr.status = 2)
        left  JOIN task as td 
                        on ( td.id = tr.task and tr.status = 3)
        group by task.id 
    

BY Shahen Babayan

Answer 2

现有查询存在一些问题。首先，您不能在任何列上将两个表连接在一起。您似乎可以加入task.id和transaction.task。第二。你应该能够通过使用具有一些条件逻辑的聚合函数来获得每个总数，例如CASE表达式：

SELECT tk.name, tk.priority,
    sum(case when tran.status = 1 then 1 else 0 end) waiting,
    sum(case when tran.status = 3 then 1 else 0 end) error,
    sum(case when tran.status = 4 then 1 else 0 end) done,
    count(*) total
FROM task AS tk
INNER JOIN transaction AS tran
    ON tk.id = tran.task
GROUP BY tk.name, tk.priority;

这种类型的查询称为PIVOT，您可以从中获取行中的值并将其转换为行。在CASE表达式中，您只能获得每个status的总计。