A2-06-07.MySQL CASE Statement


转载自:http://www.mysqltutorial.org/mysql-case-statement/

 

MySQL CASE Statement

 

Summary: in this tutorial, you will learn how to use MySQL CASE statements to construct complex conditional statements inside stored programs.

Besides the IF statement, MySQL provides an alternative conditional statement called the CASEstatement. The CASE statement makes the code more readable and efficient.

There are two forms of the CASE statements: simple and searched CASE statements.

Simple CASE statement

Let’s take a look at the syntax of the simple CASE statement:

You use the simple CASE statement to check the value of an expression against a set of unique values.

The case_expressioncan be any valid expression.  The case_expression is compared with  when_expression  in each WHEN clause e.g., when_expression_1  and when_expression_2 . If the values of the case_expression and when_expression_n are equal, the  commands in the corresponding WHEN branch executes.

In case none of the when_expression in the WHEN clause matches the value of the case_expression the commands in the ELSE clause will execute. The ELSE  clause is optional. If you omit the ELSE clause and no match found, MySQL will raise an error.

The following example illustrates how to use the simple CASE statement:

How the stored procedure works.

  • The GetCustomerShipping() stored procedure accepts customer number as an IN parameter and returns the shipping period based on the country of the customer.
  • Inside the stored procedure, first, we get the country of the customer based on the input customer number. Then, we use the simple CASE statement to compare the country of the customer to determine the shipping time. If the customer locates in USA , the shipping period is 2-day shipping . If the customer is in Canada , the shipping period is 3-day shipping . The customers from other countries have 5-day shipping .

The following flowchart demonstrates the logic of determining the shipping time.

MySQL CASE statement flowchart

The following is the test script for the stored procedure above:

Here is the output:

MySQL CASE Statement Example

Searched CASE statement

The simple CASE statement only allows you match a value of an expression against a set of distinct values. In order to perform more complex matches such as ranges, you use the searched CASEstatement. The searched CASE statement is equivalent to the IF  statement, however, its construct is much more readable.

The following illustrates the syntax of the searched CASE statement:

MySQL evaluates each condition in the WHEN clause until it finds a condition whose value is TRUE , then corresponding commands in the THEN clause will execute.

If no condition is TRUE, the command in the ELSE clause will execute. If you don’t specify the ELSEclause and no condition is TRUE , MySQL will raise an error.

MySQL does not allow you to have empty commands in the THEN or ELSE clause. If you don’t want to handle the logic in the ELSE clause while preventing MySQL from raising an error, you can put an empty BEGIN END  block in the ELSE clause.

The following example demonstrates using searched CASE statement to find customer level SILVER , GOLD or PLATINUM based on customer’s credit limit.

If the credit limit is

  • greater than 50K, then the customer is the PLATINUM customer.
  • less than 50K and greater than 10K, then the customer is the GOLD customer.
  • less than 10K, then the customer is the SILVER customer.

We can test the stored procedure by executing the following test script:

MySQL searched CASE Statement Example

In this tutorial, we have shown you how to use two forms of the MySQL CASE statements including simple CASE statement and searched CASE statement.


注意!

本站转载的文章为个人学习借鉴使用,本站对版权不负任何法律责任。如果侵犯了您的隐私权益,请联系我们删除。



 
  © 2014-2022 ITdaan.com