无法使用 mysql 和节点 js 返回选择查询值

Question

我是 node js 的新手，现在我正在尝试使用 node js 在 mysql 中设置 select 查询的返回值....我正在使用 node-mysql 包...

示例代码

var mysql =  require('mysql');
var connection =  mysql.createConnection({
    host : "localhost",
    user : "root",
    password: "root123",
    database: "testdb"
});

var retValue = undefined;

var query = connection.query('SELECT * FROM tblData;');
query
    .on('error', function(err) {
        // Handle error, an 'end' event will be emitted after this as well
    })
    .on('fields', function(fields) {
        // the field packets for the rows to follow
    })
    .on('result', function(row) {
        // Pausing the connnection is useful if your processing involves I/O
        connection.pause();
        processRow(row, function() {
            retValue = row;
        });
    })
    .on('end', function(row) {

    });

    connection.end();

    function processRow(rows)
    {
        retValue = rows;
    }

    console.log(retValue);

retValue 始终未定义。我知道这是异步调用。谁能告诉我如何为这个变量设置值。

谢谢 迪帕克

Answer 1

由于数据库查询是异步操作，因此在您调用console.log(retValue) 时，您的变量retValue 尚未设置。

var retValue;

var query = connection.query('SELECT * FROM tblData;');
query
    .on('error', function(err) {
        // Handle error, an 'end' event will be emitted after this as well
    })
    .on('fields', function(fields) {
        // the field packets for the rows to follow
    })
    .on('result', function(row) {
        // Pausing the connnection is useful if your processing involves I/O
        connection.pause();
        processRow(row);
        console.log(retValue); //retValue is now set
    })
    .on('end', function(row) {

    });

    connection.end();

    function processRow(rows)
    {
        retValue = rows;
    }

    console.log(retValue); // undefined, retValue has not been set yet

Answer 2

retvalue 仅在您放入结果部分的范围内可用。所以你可以做这样的事情来记录你的结果：

var connection =  mysql.createConnection({
    host : "localhost",
    user : "root",
    password: "root123",
    database: "testdb"
});

var retValue = undefined;

var query = connection.query('SELECT * FROM tblData;', function(err, results, fields) {
    console.log(results);
    // Here continue logic you needed with results from table
});

Answer 3

将console.log 移动到function processRow(rows) 中，使其看起来像

function processRow(rows)
{
    retValue = rows;
    console.log(retValue);
}

也变了

processRow(row, function() {
    retValue = row;
});

到

processRow(row);

第二个参数未使用。

Answer 4

我无法得到超出范围的结果...我想像这样分配并返回值

函数() 获取结果{ ... ... var retValue = 未定义；

var query = connection.query('SELECT * FROM tblData;', function(err, results, fields) { retValue = 结果； });

返回 retValue };

var ret = getResults(); console.log(ret);