如何从查询语句之外的mysql查询中获取值？

Question

这里是console.log下面的函数

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} ##

我这样调用它：

quo();

Answer 1

tl;dr一切都发生在回调中。

您对 Javascript 的异步特性感到困惑。在您的console.log(value); 调用运行时，查询尚未（必然）完成。因此，当时无法获得查询结果。

许多开发者使用这样的模式，当查询结果到达时，带有回调函数来处理下一步。

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 对象使这种东西在 node.js 中更容易阅读。但无论如何解释它们超出了 Stack Overflow 答案的范围。

Answer 2

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 */
});