[英]Joining 3 tables on sql and checking conditions
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
聯接時這三個表工作正常,但是當我嘗試添加這些修飾符時,它們不起作用
group by ClassicModels.Orders.CustomerNumber
having count(ClassicModels.Orders.CustomerNumber) < 4
首先,歡迎使用StackOverflow!
我已使用我最喜歡的工具重新格式化了SQL,如果您有興趣,可以提供一個鏈接。 我還添加了別名以幫助使其更加“可讀”。 (別名是FROM
和JOIN
子句中表名后面的小寫位。)
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
現在我們已經完成了。 讓我們添加您的GROUP BY
和HAVING
子句。
GROUP BY
子句必須具有SELECT
子句中使用的所有列。 (我不確定為什么。我還沒有查找它,但我只知道它是這樣工作的。:))
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
現在應該可以了。 然后,您只需在其中添加HAVING
子句。
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
我也在查看您的查詢,通過使用以下查詢,您可能會更快(結果更高效):
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;
它使用所謂的“公用表表達式”,並且基本上只是隔離查詢的一部分。 它可能會更高效,因為它將嘗試對更少的數據進行分組,因此可能會更快。 警惕,因為里面有很多“可能”,因為我不知道如何在MySQL數據庫中設置各種不同的東西。
祝好運!
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