MySQL局部变量 [英] MySQL local variables

问题描述

我正在尝试为查询定义和初始化MySQL变量.

I am trying to define and initialize a MySQL variable for a query.

我有以下内容:

declare @countTotal int;
SET @countTotal = select COUNT(*)
 from nGrams;

我在Netbeans中使用MySQL，它告诉我我有一个错误.我的错误在哪里?

I am using MySQL in Netbeans and it tells me I have an error. What/where is my error?

我该如何解决?

推荐答案

MySQL有两种不同类型的变量:

MySQL has two different types of variable:

• 局部变量(不是前缀的 )是强类型的，作用域是声明它们的存储程序块.请注意，如 DECLARE语法:

• local variables (which are not prefixed by @) are strongly typed and scoped to the stored program block in which they are declared. Note that, as documented under DECLARE Syntax:

DECLARE 仅在

DECLARE is permitted only inside a BEGIN ... END compound statement and must be at its start, before any other statements.

用户变量 (以@为前缀的 )被松散地键入并限制在会话范围内.请注意，它们既不需要也不能声明，只需直接使用它们即可.

user variables (which are prefixed by @) are loosely typed and scoped to the session. Note that they neither need nor can be declared—just use them directly.

因此，如果您要定义一个存储程序并确实想要一个局部变量"，则按照问题中的措辞，您需要删除@字符并确保您的DECLARE语句位于程序块的开始.否则，要使用用户变量"，请删除DECLARE语句.

Therefore, if you are defining a stored program and actually do want a "local variable", per the wording in your question, you will need to drop the @ character and ensure that your DECLARE statement is at the start of your program block. Otherwise, to use a "user variable", drop the DECLARE statement.

此外，您可能需要将查询括在括号中才能将其作为子查询执行:

Furthermore, you will either need to surround your query in parentheses in order to execute it as a subquery:

SET @countTotal = (SELECT COUNT(*) FROM nGrams);

否则，您可以使用 SELECT ... INTO :

Or else, you could use SELECT ... INTO:

SELECT COUNT(*) INTO @countTotal FROM nGrams;

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