我在mysql中有两个表,想根据另一个表的结果查询一个表,所以我写了一个函数
export function getLocations(req, res) {
const database = new Database();
database.query('select * from districts')
.then(rows => {
let appData = [];
rows.forEach(row => {
const new_database = new Database();
new_database.query(`SELECT locations.id,locations.name, IF(subscriptions.id IS NULL,0,1) as subscribed
FROM locations
LEFT JOIN subscriptions
ON (subscriptions.location_id = locations.id AND subscriptions.user_id=1)
WHERE locations.district=?`,row.id)
.then(sub_rows => {
let district=row;
district["locations"]=sub_rows;
appData.push(district);
new_database.close();
}, err => {
return new_database.close().then(() => { throw err; })
})
.catch(err => {
console.log(err);
res.status(500).json("Database Error");
})
});
res.status(200).json(appData); //final result here
database.close()
}, err => {
return database.close().then(() => { throw err; })
})
.catch(err => {
console.log(err);
res.status(500).json("Database Error");
})
}
在这里,我想基于第一个查询的每一行运行第二个查询。 我得到一个空数组作为结果。我的第一个查询正确执行,并且我记录了所有行都已返回。可能是什么问题?
答案 0 :(得分:0)
您可以通过使其异步来使其工作
rows.forEach(async row => {
const new_database = new Database();
await new_database.query(`SELECT locations.id,locations.name, IF(subscriptions.id IS NULL,0,1) as subscribed
FROM locations
LEFT JOIN subscriptions
ON (subscriptions.location_id = locations.id AND subscriptions.user_id=1)
WHERE locations.district=?`,row.id)
.then(sub_rows => {
let district=row;
district["locations"]=sub_rows;
appData.push(district);
new_database.close();
}, err => {
return new_database.close().then(() => { throw err; })
})
.catch(err => {
console.log(err);
res.status(500).json("Database Error");
})
});
您正在执行的操作是I / O,而JS是单线程的。从外行的角度来看,这意味着它不会等待,它将在您将要发出请求的地方迭代您的循环,而在请求处理期间会有一些等待。您需要告诉JS此事件是异步的。为此,您需要使用async / await
指南
答案 1 :(得分:0)
我的机器没有环境。它可能有一些错误,但是如果有的话,您可以修复它,请看以下逻辑
export function getLocations(req, res) {
const database = new Database();
const promises=[];
database.query('select * from districts')
.then(rows => {
let appData = [];
rows.forEach(row => {
promises.push(getAnotherQuery(row));
});
database.close()
}, err => {
return database.close().then(() => { throw err; })
})
.catch(err => {
console.log(err);
res.status(500).json("Database Error");
})
return Promise.all(promises).then(result)=> res.status(200).json(result); //final result here
}
getAnotherQuery=(row)=>{
return new Promise((resolve,reject)=>{
const new_database = new Database();
const appData=[]
new_database.query(`SELECT locations.id,locations.name, IF(subscriptions.id IS NULL,0,1) as subscribed
FROM locations
LEFT JOIN subscriptions
ON (subscriptions.location_id = locations.id AND subscriptions.user_id=1)
WHERE locations.district=?`,row.id)
.then(sub_rows => {
let district=row;
district["locations"]=sub_rows;
appData.push(district);
new_database.close();
resolve(appData);
}, err => {
return new_database.close().then(() => { throw err; })
})
.catch(err => {
console.log(err);
res.status(500).json("Database Error");
})
});
}