SQL：加入组功能

Question

我的桌子是：

表 1

Uid、hospital_nr、department_nr、diagnosis_nr、case_amount

我的查询： 我想知道，对于每家医院，哪个科室的三个特定诊断病例最多。

要找出这些诊断的每个部门的病例总数，我使用：

SELECT hospital_nr, department_nr, sum(case_amount) AS cases 
FROM Table_1 
WHERE diagnosis_nr = 1 OR diagnosis_nr = 3 OR diagnosis_nr = 4
GROUP BY hospital_nr, department_nr;

要找出一个部门对各个医院的最大病例数，我使用：

SELECT b.hospital_nr, max(a.sum_of_cases) AS max_sum_of_cases
FROM hdiag_data2014 AS b,
(SELECT Hospital_nr, department_nr, sum(case_amount) AS sum_of_cases 
FROM Table_1 
WHERE diagnosis_nr = 1 OR diagnosis_nr = 3 OR diagnosis_nr = 4
GROUP BY hospital_nr, department_nr) AS a
WHERE diagnosis_nr = 1 OR diagnosis_nr = 3 OR diagnosis_nr = 4
AND b.hospital_nr = a.hospital_nr 
GROUP BY b.hospital_nr;

现在我想在一个 INNER JOIN 中加入这两个表并尝试了这个：

SELECT c.hospital_nr, c.department_nr, sum(case_amount) AS cases 
FROM Table_1 AS c
INNER JOIN
    (SELECT b.hospital_nr max(a.sum_of_cases) AS max_sum_of_cases
    FROM Table_1 AS b,
        (SELECT hospital_nr, department_nr, sum(case_amount) AS sum_of_cases
        FROM Table_1 
        WHERE diagnosis_nr = 1 OR diagnosis_nr = 3 OR diagnosis_nr = 4
    GROUP BY hospital_nr, department_nr) AS a
    WHERE b.diagnosis_nr = 1 OR b.diagnosis_nr = 3 OR b.diagnosis_nr = 4
    AND b.hospital_nr = a.hospital_nr 
    GROUP BY b.hospital_nr) AS b 
ON c.cases = b.max_sum_of_cases
WHERE c.diagnosis_nr = 1 OR c.diagnosis_nr = 3 OR c.diagnosis_nr = 4
GROUP BY c.hospital_nr;

这个脚本不允许我通过这个 ON 加入，因为它说它不能将“案例”识别为列。这是为什么？我该如何改进它？ 我回答“口头查询”的第一条路径是通过 HAVING 子句工作，但这也不成功，因为它不允许我按案件数量最多的部门进行过滤。我忽略了这条替代路径中的某些内容吗？

Answer 1

我建议使用substring_index()/group_concat() 技巧：

SELECT hospital_nr,
       SUBSTRING_INDEX(GROUP_CONCAT(department_nr ORDER BY cases DESC), ',', 1) as max_department_nr
FROM (SELECT hospital_nr, department_nr, sum(case_amount) AS cases 
      FROM Table_1 
      WHERE diagnosis_nr in (1, 3, 4)
      GROUP BY hospital_nr, department_nr
     ) hd
GROUP BY hospital_nr;

还有其他方法，但是这种方法一般是 MySQL 中最简单的。

注意：这里假设 department_nr 不包含逗号。

Answer 2

您可以在 WHERE 子句中使用带有LIMIT 1 的有序相关子查询来过滤出department_nr 和最大的case_amount：

SELECT DISTICT hospital_nr, department_nr
FROM Table_1 t1
WHERE department_nr = (
    SELECT department_nr 
    FROM Table_1 t2
    WHERE t2.hospital_nr = t1.hospital_nr
      AND t2.diagnosis_nr IN (1, 3, 4)
    ORDER BY sum(case_amount) DESC
    LIMIT 1
)

如果您还需要总和，则需要再次计算：

SELECT hospital_nr, department_nr, sum(case_amount) AS cases
FROM Table_1 t1
WHERE department_nr = (
    SELECT department_nr 
    FROM Table_1 t2
    WHERE t2.hospital_nr = t1.hospital_nr
      AND t2.diagnosis_nr IN (1, 3, 4)
    ORDER BY sum(case_amount) DESC
    LIMIT 1
)
GROUP BY hospital_nr, department_nr

注意：如果两个部门的总和相同，则查询将只“选择”一个。如果您想定义在这种情况下选择哪一个，您应该在 ORDER BY 子句中添加一列（例如department_id）。

Answer 3

我认为您缺少别名“c”。在加入。其他一些别名也丢失了，所以我不确定他们应该从哪个表中提取。

SELECT c.hospital_nr, c.department_nr, c.cases 
FROM (SELECT hospital_nr, department_nr, sum(case_amount) AS cases
        FROM Table_1 
        WHERE diagnosis_nr in (1, 3, 4)
    GROUP BY hospital_nr, department_nr) AS c
INNER JOIN
    ((SELECT b.hospital_nr, max(a.sum_of_cases) AS max_sum_of_cases
    FROM hdiag_data2014) AS b,
        (SELECT hospital_nr, department_nr, sum(case_amount) AS sum_of_cases
        FROM Table_1 
        WHERE diagnosis_nr in (1, 3, 4)
    GROUP BY hospital_nr, department_nr) AS a
    WHERE b.diagnosis_nr in (1, 3, 4)
    AND b.hospital_nr = a.hospital_nr 
    GROUP BY b.hospital_nr) AS b 
ON c.cases = b.max_sum_of_cases);