我在nodejs中有这个查询。请修改代码，使其在每次迭代时查询数据库，并将每次结果存储在结果数组中。 for循环执行结束后，它将被发送到我的回调函数。当我尝试在result.push（数据）之后进行回调（结果）时，它会工作但由于for循环它将发送几乎rows.length次。
function query_database（req，callback）{ 结果= [];
class_code=req.body.class;
branch=req.body.branch;
year=req.body.year;
start_date=req.body.start_date;
end_date=req.body.end_date;
flag=false;
query='select id,rollno from student where branch="'+branch+'" and class="'+class_code+'" and year="'+year+'"';
//console.log(query);
connection.query(query, function(err, rows, fields) {
if (!err){
query_total_working_days(start_date,end_date,branch,class_code,year,function(err,data){
if(err){
}
else{
total_days=data;
for(index=0;index<rows.length;index++){
query_attendance(rows[index].id,rows[index].rollno,start_date,end_date,total_days,function(err,data){
if(err){
}
else{
result.push(data)
}
});
}
}
})
callback(null,result)
}
else
console.log('Error while performing Query.');
});
}
答案 0 :(得分：0)
您的代码可能会像这样简化：
function query_database(callback) {
let result = [];
query_all(function(rows) {
for (let i = 0; i < rows.length; i++) {
query_row(rows[i], function (data) {
result.push(data); // (2)
});
}
})
callback(result); // (1)
}
问题是你不能确定（2）在（1）之前完成。实际上，在调用所有（2）之前调用（1）。
要使其正确，您需要更改为以下代码：
function queryDatabase(callback) {
let result = [];
queryAll(function(rows) {
let i = 0;
let rowCallBack = function (data) {
result.push(data);
i++;
if (i == rows.length) {
callback(result);
} else {
queryRow(row[i], rowCallBack);
}
};
queryRow(row[i], rowCallBack);
})
callback(result); // (1)
}
答案 1 :(得分：0)
query_attence是同步的...... 这应该可以胜任。
/* global req, connection, query_total_working_days, query_attendance, callback */
// assuming req is a correct request.
var class_code = req.body.class,
branch = req.body.branch,
year = req.body.year,
start_date = req.body.start_date,
end_date = req.body.end_date,
// this `flag` is never used, why ?? you can remove it.
flag = false,
// assuming this is a query that represent a correct SQL query
query = 'select id,rollno from student where branch="' + branch + '" and class="' + class_code + '" and year="' + year + '"';
// assuming connection is a correct sql connection object
// fields is never used, you can remove it.
connection.query(query, function(err, rows, fields) {
if (err) {
return console.log('Error while performing Query.');
}
// assuming query_total_working_days is a correct « async » function.
query_total_working_days(start_date, end_date, branch, class_code, year, function(err, data) {
var total_days,
result = [],
index,
next = function (err, data) {
if (err) {
return ; // you should provide a code for this error.
}
// result is not declared, initially. So I create it above.
result.push(data);
};
if (err) {
return ; // you should provide a code for this error
}
total_days = data;
// assuming query_attendance is a correct « sync » function.
for (index = 0; index < rows.length; index++) {
// I move the `next` callback not it this loop because expression function should not be declared in loop.
query_attendance(rows[index].id, rows[index].rollno, start_date, end_date, total_days, next);
}
// assuming callback is a correct function.
callback(null, result);
});
});
query_attence是asynchrone（可能）您的问题是：callback将在所有query_attendance完成之前执行。因此，您的result数组将不包含您想要的所有条目。
所以在这种情况下。您应该使用模块async作为示例https://github.com/caolan/async/blob/v1.5.2/README.md
如果结果的顺序很重要，请在series中使用。
如果您的结果顺序不重要，为了更快，请在parallele
/* global req, connection, query_total_working_days, query_attendance, callback, async */
// assuming req is a correct request.
var class_code = req.body.class,
branch = req.body.branch,
year = req.body.year,
start_date = req.body.start_date,
end_date = req.body.end_date,
// this `flag` is never used, why ?? you can remove it.
flag = false,
// assuming this is a query that represent a correct SQL query
query = 'select id,rollno from student where branch="' + branch + '" and class="' + class_code + '" and year="' + year + '"';
// assuming connection is a correct sql connection object
connection.query(query, function(err, rows, fields) {
// fields is never used, you can remove it.
if (err) {
return console.log('Error while performing Query.');
}
// assuming query_total_working_days is a correct « async » function.
query_total_working_days(start_date, end_date, branch, class_code, year, function(err, data) {
var total_days,
parallels = [],
next = function (next) {
query_attendance(rows[index].id, rows[index].rollno, start_date, end_date, total_days, function (err, data) {
if (err) {
return ; // you should provide a code for this error
}
// result is not declared, initially. So I create it.
next(null, data);
});
};
if (err) {
return ; // you should provide a code for this error
}
total_days = data;
// assuming query_attendance is a correct « async » function.
// we use let to not set value to variable objet in order to kept the current value of `index` for each call of `next`.
for (let index = 0; index < rows.length; index++) {
parallels.push(next);
}
// assuming async is the async npm module. In this case, result order have no importance.
async.parallel(parallels, function (results) {
// assuming callback is a correct function.
callback(null, results);
});
});
});
注意：我无法测试此代码，如果您有错误，请告诉我。