MYSQL |从另一个字段中减去一个字段，但 1 个字段是由子查询创建的

Question

SELECT 
E.`employee_id`,
E.`full_name`,
LE.`no_of_leaves` AS AllocatedLeaves,
MLLT.`leave_type` AS LeaveTypeName,
(SELECT COUNT(*) FROM leave_approval WHERE employee_id = 1 AND MONTH(approval_date) = 11 GROUP BY approval_date) AS TotalLeavesTaken,
LE.`no_of_leaves` - TotalLeavesTaken AS Balance
FROM employee E
INNER JOIN leave_entitlement LE
ON E.`employee_id` = LE.`employee_id`
INNER JOIN `ml_leave_type` MLLT
ON MLLT.`ml_leave_type_id` = LE.`ml_leave_type_id`
LEFT JOIN leave_approval LA
ON E.`employee_id` = LA.`employee_id`
LEFT JOIN leave_application LAPP
ON LAPP.`application_id` = LA.`leave_application_id`
LEFT JOIN ml_leave_type MLLTLA
ON MLLTLA.`ml_leave_type_id` = LAPP.`ml_leave_type_id`

Error Code: 1054
Unknown column 'TotalLeavesTaken' in 'field list'

这是我尝试做的，但出错了..

LE.`no_of_leaves` - TotalLeavesTaken AS Balance

我不擅长数据库，我必须从另一个字段中减去一个字段，第一个字段没问题，但第二个字段是作为子查询生成的。我不想再次运行子查询，是否有可能在不再次使用子查询的情况下进行减法？？

Answer 1

您需要将计算总叶数的计算移到派生表中，而不是子查询，以便在查询的其他地方再次使用它。

此外，子查询本身看起来有问题，您确定要将其与实际员工相关联吗？而且我不确定你对 group by 的意图，所以我省略了它。

SELECT 
  E.`employee_id`,
  E.`full_name`,
  LE.`no_of_leaves` AS AllocatedLeaves,
  MLLT.`leave_type` AS LeaveTypeName,
  lt.TotalLeavesTaken as TotalLeavesTaken,
  LE.`no_of_leaves` - lt.TotalLeavesTaken AS Balance
FROM employee E
  LEFT JOIN
  (
    SELECT COUNT(*) AS TotalLeavesTaken
     FROM leave_approval 
     WHERE employee_id = E.`employee_id` AND MONTH(approval_date) = 11 
   ) AS lt -- GROUP BY approval_date doesn't make sense to me
  INNER JOIN leave_entitlement LE
    ON E.`employee_id` = LE.`employee_id`
  INNER JOIN `ml_leave_type` MLLT
    ON MLLT.`ml_leave_type_id` = LE.`ml_leave_type_id`
  LEFT JOIN leave_approval LA
    ON E.`employee_id` = LA.`employee_id`
  LEFT JOIN leave_application LAPP
    ON LAPP.`application_id` = LA.`leave_application_id`
  LEFT JOIN ml_leave_type MLLTLA
    ON MLLTLA.`ml_leave_type_id` = LAPP.`ml_leave_type_id`

Answer 2

试试这个：

SELECT E.employee_id, E.full_name, LE.no_of_leaves AS AllocatedLeaves,
       MLLT.leave_type AS LeaveTypeName,
      (LE.no_of_leaves - SUM(CASE WHEN LA.employee_id = 1 AND MONTH(LA.approval_date) = 11 THEN 1 ELSE 0 END)) AS balance
FROM employee E
INNER JOIN leave_entitlement LE ON E.employee_id = LE.employee_id
INNER JOIN `ml_leave_type` MLLT ON MLLT.ml_leave_type_id = LE.ml_leave_type_id
LEFT JOIN leave_approval LA ON E.employee_id = LA.employee_id
LEFT JOIN leave_application LAPP ON LAPP.application_id = LA.leave_application_id
LEFT JOIN ml_leave_type MLLTLA ON MLLTLA.ml_leave_type_id = LAPP.ml_leave_type_id
GROUP BY E.employee_id;

编辑

如果您想按员工计算休假余额

SELECT E.employee_id, E.full_name, LE.no_of_leaves AS AllocatedLeaves,
       MLLT.leave_type AS LeaveTypeName, LA.TotalLeavesTaken, 
      (LE.no_of_leaves - LA.TotalLeavesTaken) AS balance
FROM employee E
INNER JOIN leave_entitlement LE ON E.employee_id = LE.employee_id
INNER JOIN ml_leave_type MLLT ON MLLT.ml_leave_type_id = LE.ml_leave_type_id
LEFT JOIN (SELECT LA.employee_id, COUNT(1) As TotalLeavesTaken 
           FROM leave_approval LA 
           WHERE MONTH(LA.approval_date) = 11 
           GROUP BY LA.employee_id
         ) AS LA ON E.employee_id = LA.employee_id
LEFT JOIN leave_application LAPP ON LAPP.application_id = LA.leave_application_id
LEFT JOIN ml_leave_type MLLTLA ON MLLTLA.ml_leave_type_id = LAPP.ml_leave_type_id
GROUP BY E.employee_id;