从Node.js中的SQL查询返回变量

Question

I have MySQL query in Node.js where I'm trying to return "variable" and assign in to getVarible OUTSIDE my query. Is it possible to do in this case? If so, how?

Here is my code :

var sqlQuery = connection.query(result, function(err, rows, fields) {
        var count = rows.length;
        if(count === 1){
            var variable = rows[0].item;
            return variable;
        }            
    }
});

var getVariable = variable;

Answer 1

If you declare getVariable before your query call, you should be able to access it once your query return, within your cacllback fuction. You can try something like this:

var getVariable = {};

var sqlQuery = connection.query(result, function(err, rows, fields) {
        var count = rows.length;
        if(count === 1){
            getVariable = rows[0].item;
        }            
    }
});

However keep in mind that query execution is asynchronous, so your variable will be empty until your callback is executed.

So I would suggest you all logic where you use that variable to be executed within callback or use a promise of async framework.

Answer 2

Your connection.query does not return what you think it returns ie return variable; statement has no effect.

Checkout the API with the usage of query object.

var query = connection.query('SELECT * FROM posts');
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() {
      connection.resume();
    });
  })
  .on('end', function() {
    // all rows have been received
  });

Alternatively you can have a short hand to handle the success, error by using the second parameter to connection.query . This is a function with signature function (error, results, fields) which does the same thing as above API.

connection.query(sqlQueryStr, function (error, results, fields) {})

The call to connection.query is asynchronous and the function passed as the second argument is executed only after the results are available ie the query has executed.