SQL 服務器組記錄基於另一個列

Question

我正在處理包含員工數據的表。 該表具有基於部門和年份的歷史員工記錄，如下所示：

現在我想根據 EmployeeId、Department 合並記錄，並像這樣獲取 Min FromYear 和 Max ToYear：

我嘗試使用查詢：

Select EmployeeId, Department, MIN(FromYear), MAX(ToYear)
from Employee
GROUP BY EmployeeId, Department

但是對於 ID 為 3 的員工，此查詢失敗，因為它只返回 2 行：

我在這里添加了類似的結構和查詢： http://sqlfiddle.com/#!9/6f1e53/5

任何幫助將不勝感激！

Answer 1

這是一個差距和孤島問題。 使用lag()和累積和識別島嶼。 然后聚合：

select employeeid, department, min(fromyear), max(toyear)
from (select e.*,
             sum(case when prev_toyear >= fromyear - 1 then 0 else 1 end) over (partition by employeeid order by fromyear) as grp
      from (select e.*,
                   lag(toyear) over (partition by employeeid, department order by fromyear) as prev_toyear
            from employee e
           ) e
      ) e
group by employeeid, department, grp
order by employeeid, min(fromyear);

這是一個 db<>fiddle。

Answer 2

你也可以使用自我加入

select a.employeeid, min(a.fromyear), max(b.toyear) from emp a
在 a.employeeid=b.employeeid 組上由 a.employeeid 內部加入 emp b