案例表达与案例陈述 [英] Case Expression vs Case Statement

问题描述

Case Expression 和MySQL中的案例声明?什么时候可以使用它们?与另一种相比，使用它们有什么好处?

What is the difference between a Case Expression and a Case Statement in MySQL? When can they be used, and what are the benefits of using one over the other?

Case Statement语法:

Case Statement syntax:

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

Case Expression语法:

Case Expression syntax:

CASE 
  WHEN [condition] THEN result 
  [WHEN [condition] THEN result ...] 
  [ELSE result] 
END

这些看起来几乎相同，但是对Case Statements的最初描述是The CASE statement for stored programs implements a complex conditional construct.

These look almost identical, but the initial description for Case Statements is that The CASE statement for stored programs implements a complex conditional construct.

那么，在存储程序中使用一个而不在普通查询中使用它的显着区别是吗?我在正在使用的查询中尝试了此方法，但失败了- sqlfiddle .如果是这种情况，为什么不只在存储程序中使用Case Expression?

So is the significant difference that one is used in stored programs and not usable in normal queries? I tried this out on a query I was playing with and it failed - sqlfiddle. If this is the case though, why not just use the Case Expression in a stored program?

还有其他语法上的差异，因为它们似乎是相同的吗?

Are there any other syntactical differences to be aware of, since they seem to be identical?

推荐答案

CASE表达式根据某个条件求值，即，它用于根据一组条件对一组结果之一求值.
示例:

The CASE expression evaluates to a value, i.e. it is used to evaluate to one of a set of results, based on some condition.
Example:

SELECT CASE
    WHEN type = 1 THEN 'foo'
    WHEN type = 2 THEN 'bar'
    ELSE 'baz'
END AS name_for_numeric_type
FROM sometable`

CASE语句根据某些条件执行一组语句中的一个.
示例:

The CASE statement executes one of a set of statements, based on some condition.
Example:

CASE
    WHEN action = 'update' THEN
        UPDATE sometable SET column = value WHERE condition;
    WHEN action = 'create' THEN
        INSERT INTO sometable (column) VALUES (value);
END CASE

您会看到它们的相似之处，但是语句不会求值，并且可以单独使用，而表达式必须是表达式的一部分，例如查询或作业.您无法在查询中使用该语句，因为查询不能包含语句，因此仅需要对某些内容求值的表达式(在某种程度上，查询本身就是一条语句)，例如SELECT CASE WHEN condition THEN UPDATE table SET something; END CASE没有道理.

You see how they are similar, but the statement does not evaluate to a value and can be used on its own, while the expression needs to be a part of an expression, e.g. a query or an assignment. You cannot use the statement in a query, since a query cannot contain statements, only expressions that need to evaluate to something (the query itself is a statement, in a way), e.g. SELECT CASE WHEN condition THEN UPDATE table SET something; END CASE makes no sense.

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