Question

我有这样一个测试表：Table Rows setup

我的目标是尝试获取在当前部门之前建立的部门数。我的SQL是：

SELECT A.Department, IFNULL(COUNT(*), 0) 
FROM Departments A
INNER JOIN Departments B ON B.YearOfEstablishment < A.YearOfEstablishment
GROUP BY Department
ORDER BY COUNT(*);

但是我同时尝试了 LEFT JOIN 和 INNER JOIN ，最后找到的第一个部门永远不会返回，因为我可以假设一片空白。尽管有IFNULL，但部门未显示。

我在这里做什么错了？

Answer 1

我认为这是您需要的查询：

SELECT A.Department, COUNT(B.Department)
FROM Departments A
LEFT JOIN Departments B ON B.YearOfEstablishment < A.YearOfEstablishment
GROUP BY A.Department
ORDER BY 2;

请参见 this db fiddle demo 。

| Department        | cnt |
| ----------------- | --- |
| Office Management | 0   |
| Business          | 1   |
| Sales Management  | 2   |
| ComputerScience   | 3   |
| Liberal Arts      | 4   |
| Farming           | 4   |
| Communications    | 6   |
| Digital Science   | 7   |

NB：@fifonik指出，IFNULL不需要，因为COUNT在没有可用记录时已经返回了0。

Answer 2

在MySQL 8+中，您可以使用rank()或row_number()更好地做到这一点：

SELECT d.Department,
       ROW_NUMBER() OVER (ORDER BY YearOfEstablishment) - 1 as seqnum
FROM Departments d
ORDER BY seqnum;

没有关系，这与您的查询相同。这样做可能更好：

SELECT d.Department,
       RANK() OVER (ORDER BY YearOfEstablishment) - 1 as seqnum
FROM Departments d
ORDER BY seqnum;

这应该是您要查找的数量。

Answer 3

您还可以执行以下操作：

SELECT
      A.Department
    , A.YearOfEstablishment
    , COUNT(*) - 1
FROM
    Departments A
    INNER JOIN Departments B ON (
           A.id = B.id
        OR B.YearOfEstablishment < A.YearOfEstablishment
    )
GROUP BY
    Department
ORDER BY
    COUNT(*);

尽管使用了IFNULL（MySQL），SQL查询也不返回NULL值

3 个答案: