多个内部联接-MySQL

Question

I have 2 tables, Task and Transaction , which look like this:

Task

Transaction

I would like to create a query which return information from both tables.

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

where:

• 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

I tried with INNER JOIN but I get a wrong result:

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;

Could you please help me creating this query? I'm getting the columns waiting, error, done, total with a count of all the transactions. Instead it should get the number of transaction WHERE task.id = transaction.task and 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

A few issues with your existing query. First, you aren't joining your two tables together on any column. It looks like you can join on task.id and transaction.task . Second. you should be able to get each of the total by using an aggregate function with some conditional logic, like a CASE expression:

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;

This type of query is known as a PIVOT, it's where you take the values from your rows and converts them into rows. Inside the CASE expression, you'll only get the total for each status .