Question

这是函数，见下面的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} ##

我这样调用它：

quo();

Answer 1

tl; dr 一切都在回调中发生。

您正在绊倒Javascript的异步性质。当您的console.log(value);调用运行时，查询不会（必然）完成。因此，当时无法获得查询结果。

许多开发人员使用这样的模式，使用回调函数来处理查询结果到达时的下一步。

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个对象使得node.js中的这类内容更容易阅读。但无论如何解释它们都超出了Stack Overflow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 */
});

如何从查询语句之外的mysql查询中获取值？

2 个答案: