Question

我构建了这个查询以向我展示分支机构中有多少员工和客户，它给了我重复项，因此我需要对它们进行分组。但是，当我添加组时，查询无效。任何人都知道为什么？

  SELECT B.branchID, B.branchName, COUNT(E.staffNo) AS "No. Of Employees", COUNT(C.customerID) AS "No. Of Customers"
        FROM Branch B, Employee E, Customer C
        WHERE E.employeeBranchID = B.branchID
        AND B.branchID = C.customerBranchID
        GROUP BY B.branchID;

Answer 1

这个查询最好写成：

SELECT B.branchID, B.branchName, COUNT(E.staffNo) AS "No. Of Employees", COUNT(C.customerID) AS "No. Of Customers"
FROM Branch B JOIN
     Employee E
     ON E.employeeBranchID = B.branchID JOIN
     Customer C
     ON B.branchID = C.customerBranchID
GROUP BY B.branchID, B.branchName;

如果您处于ANSI模式，那么MySQL的行为与大多数其他数据库一样，并且会返回错误，因为B.branchName是非聚合的。

注意：

使用显式JOIN语法。切勿在{{1}}子句中使用逗号。
所有非汇总列都在FROM。
如果GROUP BY被声明为branchId上的主要密钥或唯一密钥，则原始查询符合ANSI标准。但是，大多数数据库仍然不接受查询。

Answer 2

SELECT B.branchID, B.branchName, COUNT(E.staffNo) AS "No. Of Employees", COUNT(C.customerID) AS "No. Of Customers"
    FROM Branch B, Employee E, Customer C
    WHERE E.employeeBranchID = B.branchID
    AND B.branchID = C.customerBranchID
    GROUP BY B.branchID, B.branchName;

大多数数据库需要按项目列表==在聚合函数之前选择项目列表

Answer 3

SELECT B.branchID，B.branchName，COUNT（E.staffNo）AS＆＃34; No。员工＆＃34;，COUNT（C.customerID）AS＆＃34; No。客户＆＃34; FROM Branch B在E.employeeBranchID = B.branchID上离开加入Employee E在B.branchID = C.customerBranchID上加入Customer C GROUP BY B.branchID;

使用group by时，SQL Query无法运行

3 个答案: