SelectClassicModels.Orders.CustomerNumber,
ClassicModels.CUSTOMERS.CUSTOMERNAME,
ClassicModels.Employees.LASTNAME,
ClassicModels.Employees.firstNAME,
ClassicModels.employees.EmployeeNumber
from ClassicModels.Orders
join
ClassicModels.Customers
on ClassicModels.Orders.CustomerNumber = ClassicModels.Customers.CUSTOMERNUMBER
join
ClassicModels.EMPLOYEES
on ClassicModels.Employees.EMPLOYEENUMBER = ClassicModels.CUSTOMERS.SalesRepEmployeeNumber
这三个表在连接时工作得很好,但是当我尝试添加这些修饰符时,它们不起作用
group by ClassicModels.Orders.CustomerNumber
having count(ClassicModels.Orders.CustomerNumber) < 4
最佳答案
首先,欢迎来到 StackOverflow!
我已经使用我最喜欢的工具重新格式化了您的 SQL,如果您有兴趣,我可以提供一个链接。我还添加了别名以帮助使其更“可读”。 (别名是 FROM
和 JOIN
子句中表名后的小写位。)
SELECT orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
FROM CLASSICMODELS.ORDERS orders
JOIN CLASSICMODELS.CUSTOMERS customers
ON orders.CustomerNumber = customers.CustomerNumber
JOIN CLASSICMODELS.EMPLOYEES employees
ON employees.EmployeeNumber = customers.SalesRepEmployeeNumber
现在我们已经完成了。让我们添加您的 GROUP BY
和 HAVING
子句。
GROUP BY
子句中必须包含 ALL 在 SELECT
子句中使用的列。 (我不确定为什么。我没有查过,但我只知道它是这样工作的。:))
SELECT orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
FROM CLASSICMODELS.ORDERS orders
JOIN CLASSICMODELS.CUSTOMERS customers
ON orders.CustomerNumber = customers.CustomerNumber
JOIN CLASSICMODELS.EMPLOYEES employees
ON employees.EmployeeNumber = customers.SalesRepEmployeeNumber
GROUP BY orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
现在应该可以了。然后,您只需在其中添加 HAVING
子句即可。
SELECT orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
FROM CLASSICMODELS.ORDERS orders
JOIN CLASSICMODELS.CUSTOMERS customers
ON orders.CustomerNumber = customers.CustomerNumber
JOIN CLASSICMODELS.EMPLOYEES employees
ON employees.EmployeeNumber = customers.SalesRepEmployeeNumber
GROUP BY orders.CustomerNumber,
customers.CustomerName,
employees.LastName,
employees.FirstName,
employees.EmployeeNumber
HAVING COUNT(orders.CustomerNumber) < 4
我也在查看您的查询,您可能通过使用这样的查询获得更快(和更有效的结果):
WITH CUSTOMERSWITHLESSTHANFOURORDERS
AS
(
SELECT CUSTOMERNUMBER
FROM CLASSICMODELS.ORDERS
GROUP BY CUSTOMERNUMBER
HAVING COUNT(CUSTOMERNUMBER) < 4
)
SELECT O.CUSTOMERNUMBER,
C.CUSTOMERNAME,
E.LASTNAME,
E.FIRSTNAME,
E.EMPLOYEENUMBER
FROM CUSTOMERSWITHLESSTHANFOURORDERS O
JOIN CLASSICMODELS.CUSTOMERS C
ON O.CUSTOMERNUMBER = C.CUSTOMERNUMBER
JOIN CLASSICMODELS.EMPLOYEES E
ON E.EMPLOYEENUMBER = C.SALESREPEMPLOYEENUMBER;
它使用所谓的“公用表表达式”,基本上只是隔离了查询的一部分。它可能会更有效率,因为它会尝试对更少的数据进行分组,因此它可能会更快。小心,因为里面有很多“可能”,因为我不知道你的 MySQL 数据库中是如何设置各种不同的东西的。
祝你好运!
关于mysql - 在sql上连接3个表并检查条件,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/35347676/