从连接表中选择每组的最新数据

Question

我必须从两个表中选择数据，查询如下。

SELECT
   accounts.firstname, accounts.mobileno, accounts.address, todotasks.lastmodified
FROM accounts
JOIN todotasks ON accounts.imeino = todotasks.assignedto
WHERE status = 'Completed'

现在我希望从表格中显示最新的修改时间。

Answer 1

您可以使用ORDER BY todotasks.lastmodified DESC -

SELECT
   accounts.firstname, accounts.mobileno, accounts.address, todotasks.lastmodified
FROM accounts
JOIN todotasks ON accounts.imeino = todotasks.assignedto
WHERE status = 'Completed'
AND todotasks.lastmodified = (SELECT lastmodified FROM todotasks ORDER BY lastmodified DESC LIMIT 1)

Answer 2

在我看来，您只想对每个帐户进行分组。鉴于我们无法使用您的完整表架构，以下是最佳猜测：

SELECT
   accounts.firstname, accounts.mobileno, accounts.address, max(todotasks.lastmodified)
FROM accounts
JOIN todotasks ON accounts.imeino = todotasks.assignedto
WHERE status = 'Completed'
GROUP BY accounts.imeino

此处假设imeino每个帐户都是唯一的，firstname，mobileno和address不会因每个唯一的imeino而改变。

Answer 3

您只需group您的数据，并使用MAX函数选择最新日期：

SELECT a.firstname, a.mobileno, a.address, MAX(t.lastmodified) AS lastmodified
FROM accounts a
JOIN todotasks t ON a.imeino = t.assignedto
WHERE status = 'Completed'
GROUP BY a.firstname, a.mobileno, a.address

修改

SELECT a.firstname, a.mobileno, a.address, MAX(t1.lastmodified) AS lastmodified
FROM accounts a
JOIN todotasks t1 ON a.imeino = t1.assignedto
WHERE status = 'Completed' AND 
      NOT EXISTS(SELECT * FROM todotasks t2 WHERE t2.assignedto = t1.assignedto AND
                                                  t2.status <> 'Completed' AND
                                                  t2.lastmodified > t1.lastmodified 
                                                --AND t2.TaskNo = t1.TaskNo)
GROUP BY a.firstname, a.mobileno, a.address