从条件表获取WHERE子句的条件 [英] Getting conditions for WHERE clause from a table of criterias

问题描述

我创建了以下简单的DataModel:

我用以下数据填充了表格:

And I filled the tables with the following Data:

1)表客户

INSERT INTO test.customer
(CustomerName, Country, RegistrationDate)
VALUES 
("Customer A","DE","2015-05-03"),
("Customer B","US","2015-07-25"), 
("Customer C","US","2016-02-15"), 
("Customer D","DE","2017-09-21"), 
("Customer E","AU","2018-12-07");

2)表订单

INSERT INTO test.orders
(idCustomer, PaymentMethod, OrderDate, OrderValue)
VALUES 
("1","CreditCard","2015-05-04","500"),
("1","PayPal","2015-11-18","200"), 
("3","PayPal","2017-09-04","300"), 
("2","Invoice","2018-04-30","100");

3)表条件

INSERT INTO test.criterias
(Country, MinimumOrderValue)
VALUES 
("DE","300"),
("US","150"), 
("AU","200");

然后，我根据条件Country和OrderValue创建了一个查询以获取Customers及其Orders:

Afterwards, I created a query to get Customers and their Orders based on the criterias Country and OrderValue:

SELECT 
test.customer.idCustomer, CustomerName, Country,
test.orders.OrderValue

FROM test.customer
LEFT JOIN test.customer_info ON test.customer.idCustomer = test.customer_info.idCustomer
LEFT JOIN test.orders ON test.customer.idCustomer = test.orders.idCustomer

WHERE 
Country = "US" AND OrderValue >= "150"
OR Country = "DE" AND OrderValue >= "300" 
OR country = "AU" AND OrderValue >= "200";

到目前为止，所有这些都工作正常.

All this works fine so far.

但是，与其在SQL查询的WHERE子句中不包含这些条件，不如将它们存储在一个名为Criterias的单独表中.然后，查询应该从该表中获取数据，并像上面的查询中一样完全应用它们.

However, instead of having the criterias in the WHERE clause within the SQL query they should be stored in a seperate table called Criterias. The query should then get the datas from this table and apply them exactly like it does now in the query above.

要实现此目的，我需要在代码中进行哪些更改?

What do I have to change in my code to achieve this?

推荐答案

您只需join表或在where子句中使用子查询.

You would just join the tables or use a subquery in the where clause.

后者更易于键入，因此:

The latter is easier to type, so:

WHERE EXISTS (SELECT 1
              FROM Criterias cr
              WHERE cr.Country = c.Country AND
                    cr.MinOrderValue <=  o.OrderValue
             )

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