條件linq查詢

Question

將此SQL轉換為linq的最佳方法是什么？

我之前已經基於這樣的sql在數據庫中創建了視圖，然后使用linq查詢該視圖。 但是我想知道是否還有其他方法。

SQL查找任務的分配對象。 任務表包含三個外鍵列，分配給這些外鍵的列可能來自Department，Position或Person。 只允許一個。

SQL：

SELECT   id,
         title,
         assigned_to = (case
            when idAssignedDepartment is not null then (select description from department where id = idAssignedDepartment)
            when idAssignedPosition is not null then (select description from position where id = idAssignedPosition )
            when idAssignedPerson is not null then (select fullname from person where id = idAssignedPerson )               
            end)
FROM     task

使用LinqToEF

Answer 1

您可以這樣寫：

var q = from t in task
        from dep in department.Where(x => x.id == t.idAssignedDepartment).DefaultIfEmpty()
        from pos in position.Where(x => x.id == t.idAssignedPosition).DefaultIfEmpty()
        from per in person .Where(x => x.id == t.idAssignedPerson).DefaultIfEmpty()
        select new
        {
            t.id,
            t.title,
            assigned_to = t.idAssignedDepartment != null ? dep.description :
                          t.idAssignedPosition != null ? pos.description :
                          t.idAssignedPerson != null ? per.fullname : 
                          null
        };