如何从查询语句之外的mysql查询中获取值? [英] How to get a value from mysql query outside the query statement?

问题描述

这是该功能，请参见console.log

Here is the function see below console.log

function quo (value){
    value = connection.query(
       'SELECT role from `roles` where `id` = 1' , 
       function (error, results, fields) {
           if (error) throw error;
           console.log('The role is: ', results[0].role);// result here The role is : admin
          });
    console.log(value);
}

这里的结果是查询对象Query {domain:null，_events:{error:.. etc} ##

The result here is query object Query {domain:null,_events:{error: .. etc} ##

我这样调用:

quo();

推荐答案

tl; dr 一切都发生在回调中.

tl;dr everything happens in a callback.

您正在绊倒Javascript的异步特性.在您的console.log(value);调用运行时，该查询尚未(必须)完成.因此，当时无法获得查询结果.

You're tripping up on the asynchronous nature of Javascript. By the time your console.log(value); call runs, the query is not (necessarily) completed. So, there's no way for the result of the query to be available at that time.

许多开发人员使用这样的模式，并在查询结果到来时使用回调函数来处理下一步.

Many developers use a pattern like this, with a callback function to handle the next step when the query result arrives.

function quo (success){
    value = connection.query(
       'SELECT role from `roles` where `id` = 1' , 
       function (error, results, fields) {
           if (error) throw error;
           console.log('The role is: ', results[0].role);
           success (results[0].role);
       });
}

quo (function (role) {
   console.log(role);
   /* do something useful with the role that came back from the query */
});

Promise对象使这种事情在node.js中更易于阅读.但是无论如何，对它们的解释超出了堆栈溢出答案的范围.

Promise objects make this sort of thing easier to read in node.js. But explaining them is beyond the scope of a Stack Overflow answer anyhow.

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