mysql - 如何限制每个用户 ID 一行

Question

我有一张名为“employeetimesheets”的表格:

empsheet_id|employee_id|timesheet_status|last_update

该表允许经理访问所有员工的考勤表。一名员工可以有多个考勤表。我想仅显示每个员工的最新条目。我读过in the manual我必须编写一个分组最大子查询和带有inner join的left join，但我不确定如何在这里进行操作。

到目前为止，这是我的查询:

$sqlempsheets="SELECT * FROM employeetimesheets JOIN employees ON employeetimesheets.employee_id=employees.employee_id WHERE employeetimesheets.timesheet_status='Pending Approval'";
$resultempsheets=mysqli_query($db,$sqlempsheets);

Answer 1

试试这个:

select *
from employeetimesheets t
join (
    select employee_id,
        max(empsheet_id) as empsheet_id
    from employeetimesheets
    group by employee_id
    ) t2 on t.employee_id = t2.employee_id
    and t.empsheet_id = t2.empsheet_id
join employees e on t.employee_id = e.employee_id
where t.timesheet_status = 'Pending Approval';

或者使用左连接:

select t.*, e.*
from employeetimesheets t
left join employeetimesheets t2 on t.employee_id = t2.employee_id
    and t.empsheet_id < t2.empsheet_id
join employees e on t.employee_id = e.employee_id
where t.timesheet_status = 'Pending Approval'
    and t2.employee_id is null;