MSSQL实现复杂多重条件判断（mssql 多重判断）

SQL Server 是 a popular database server, used by organizations around the world. This comprehensive and powerful system allows organizations to query and analyze data quickly, allowing for greater decision making efficiency. However, sometimes the queries become too complex for SQL Server to handle, as it uses rigid structure, meaning that multi-condition queries must be built carefully.

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Fortunately, MSSQL (Microsoft SQL Server) makes it possible to implement complex logic with multiple conditionals. It is done through a T-SQL (Microsoft Transact-SQL), batch of queries, which can be used to create more complex conditionals. The most common way of implementing multiple conditionals is through a case statement.

Let’s take an example of a query to determine the total amount due for a group of customers based on their credit rating. The query would be designed as follows:

“`sql

SELECT

customerID,

(CASE

WHEN creditRating = ‘A’

THEN amountDue * 0.90

WHEN creditRating = ‘B’

THEN amountDue * 0.8

WHEN creditRating = ‘C’

THEN amountDue * 0.7

ELSE

amountDue

END) AS totalAmountDue

FROM

Customer

“`

In the example above, the CASE statement is evaluating the customer’s credit rating and calculating a total amount due based on that information. The case statement is evaluated from top to bottom until the statement that matches the condition is found.

The case statement is a powerful feature in MSSQL, as it enables users to easily implement complex logic with multiple conditions. The flexibility of the case statement gives users the ability to integrate variables and make quick decisions when processing data. It also provides a more organized way of dealing with complex data.

MSSQL also enables users to use variables to store the result of a single CASE statement, avoiding the need to execute multiple CASE statements. This is done by assigning a result to a variable and then using that variable in the same CASE statement.

For example, a query can be written to assign a value to a variable based on a customer’s credit rating, and then use that variable to calculate the total amount due. This can be done using the following code:

“`sql

DECLARE @creditRating varchar(2)

SELECT

@creditRating = Customer.creditRating

FROM

Customer WHERE Customer.customerID = @custID

SELECT

customerID,

(CASE

WHEN @creditRating = ‘A’

THEN amountDue * 0.90

WHEN @creditRating = ‘B’

THEN amountDue * 0.8

WHEN @creditRating = ‘C’

THEN amountDue * 0.7

ELSE

amountDue

END) AS totalAmountDue

FROM

Customer

MSSQL’s case statement is an extremely useful tool for implementing complex logic with multiple conditionals. It provides users with the ability to easily create and save variables, as well as to process and analyze data quickly, aiding in decision making efficiency. Thanks to its flexibility, multi-condition queries can be written in a more organized manner, making them easier to read and understand. 

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