MySQL CASE 是如何工作的?

How does MySQL CASE work?(MySQL CASE 是如何工作的?)

问题描述

我知道SQL的CASE语法如下:

I know that SQL's CASE syntax is as follows:

CASE
    WHEN search_condition THEN statement_list
    [WHEN search_condition THEN statement_list] ...
    [ELSE statement_list]
END CASE

但是，我不明白这是如何工作的，可能是因为我将其视为一个 if 语句.

However, I don't understand how this works, possibly because I'm thinking about it as about an if statement.

如果我在表 user_role 中有一个字段，例如，其中包含Manager"、Part Time"等名称，我如何生成字段 role_order 根据角色使用不同的数字.在本例中，如果 user_role = 'Manager' then role_order = 5".

If I have a field in table user_role, for example, which contains names like "Manager", "Part Time" etc., how do I generate a field role_order with a different number depending on the role. In the case of this example, "if user_role = 'Manager' then role_order = 5".

请注意，我正在寻找一个教一个人如何钓鱼的答案，而不是给一个人一条鱼的答案.

Please note I am looking for a teach a man how to fish answer rather than give a man a fish answer.

推荐答案

CASE 更像是一个 switch 语句.它有两种您可以使用的语法.第一个让你可以使用任何你想要的比较语句:

CASE is more like a switch statement. It has two syntaxes you can use. The first lets you use any compare statements you want:

CASE 
    WHEN user_role = 'Manager' then 4
    WHEN user_name = 'Tom' then 27
    WHEN columnA <> columnB then 99
    ELSE -1 --unknown
END

第二种样式适用于您只检查一个值的情况，并且更加简洁:

The second style is for when you are only examining one value, and is a little more succinct:

CASE user_role
    WHEN 'Manager' then 4
    WHEN 'Part Time' then 7
    ELSE -1 --unknown
END

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