node.js 操作postgresql數據庫（2）

在上一篇 node.js操作postgresql數據庫中，還有很多需要進行完善的地方，而這篇文章主要完善以下幾點：
1）將查詢的結果返還給客戶端
2）訪問數據庫的正常退出
3）返回值的問題


1）將查詢的結果返還給客戶端：
   一般的查詢都需要將結果展示出來，否則查詢將顯得沒有意義。在node.js操作postgresql數據庫中，如何將查詢的數據返回回來呢？

  (1)  select.js

function select(client,selectSQLString,callback)
{
    client.query(selectSQLString,  function selectCb(error, results)
    {
        console.log("in select callback function\n");
        if (error)
        {
            console.log('GetData Error: ' + error.message),
            client.end();
            return;
        }
         //在執行完查詢以後，結果集被存放在results中,你可以使用console.log(results)打印出來看看
       if(results.rowCount > 0)
        {
            callback(results);  
        }
    });
}

exports.select = select;

 (2) client.js

var select = require('./select');
var pg = require('pg');

var conString = "tcp://postgres:postgres@localhost/my";
var client = new pg.Client(conString);
selectSQLString = 'select * from teacher';

client.connect(function(error, results) {
    if(error){
        console.log('ClientConnectionReady Error: ' + error.message);
        client.end();
        return;
    }
    console.log('connection success...\n');
    select.select(client,selectSQLString,function(result){  
        console.log(result);
    });
});

  //採用回調函數的形式來獲取select.js文件中的查詢結果

  執行結果爲：

     connection success...

      in select callback function

      { command: 'SELECT',
        rowCount: 4,
        oid: NaN,
        rows:
        [ { id: '1', name: 'aaa', pwd: '111' },
          { id: '2', name: 'bbb', pwd: '222' },
          { id: '3', name: 'ccc', pwd: '333' },
          { id: '4', name: 'ddd', pwd: '444' } ] }

2) 訪問數據庫的正常退出：

      由於node.js的特性，若直接在調用select函數之後就關閉連接，結果可能就和我們預想的不一樣了：

  select.js不變,client.js 如下:

var select = require('./select');
var pg = require('pg');

var conString = "tcp://postgres:postgres@localhost/my";
var client = new pg.Client(conString);
selectSQLString = 'select * from teacher';

client.connect(function(error, results) {
    if(error){
        console.log('ClientConnectionReady Error: ' + error.message);
        client.end();
        return;
    }

    console.log('connection success...\n');

    select.select(client,selectSQLString,function(result){
        console.log(result);
    });


    client.end();
    console.log('Connection closed.\n');
});

   運行結果：

              connection success...

                 Connection closed.

    可以看出，並沒有將查詢的結果返回回來，連接就關閉了。這是因爲node.js執行到查詢的時候，採用非阻塞的方式，直接跳過執行後面的語句，當查詢執行完畢和調用相應的回調函數。

正確的處理方式爲：client.js:

var select = require('./select');
var pg = require('pg');

var conString = "tcp://postgres:postgres@localhost/my";
var client = new pg.Client(conString);
selectSQLString = 'select * from teacher';

client.connect(function(error, results) {
    if(error){
        console.log('ClientConnectionReady Error: ' + error.message);
        client.end();
        return;
    }

    console.log('connection success...\n');

    select.select(client,selectSQLString,function(result){
        console.log(result);
        client.end();
        console.log('Connection closed.\n');
    });
});

運行結果：

 connection success...

 in select callback function
 { command: 'SELECT',
   rowCount: 4,
   oid: NaN,
   rows:
    [ { id: '1', name: 'aaa', pwd: '111' },
      { id: '2', name: 'bbb', pwd: '222' },
      { id: '3', name: 'ccc', pwd: '333' },
      { id: '4', name: 'ddd', pwd: '444' } ] }
 Connection closed.

3)  返回值的問題:大多數情況下，函數都有返回值

   select.js

function select(client,selectSQLString,callback)
{
    var content = 'select beginning\n';
    client.query(selectSQLString,  function selectCb(error, results)
    {
        console.log("in select callback function");
        if (error)
        {
            console.log('GetData Error: ' + error.message),
            client.end();
            return;
        }
        if(results.rowCount > 0)
        {
            callback(results);
        }
    });

    content += 'select end!\n';
    return content;
}

exports.select = select;

client.js

var select = require('./select');
var pg = require('pg');

var conString = "tcp://postgres:postgres@localhost/my";
var client = new pg.Client(conString);
selectSQLString = 'select * from teacher';

client.connect(function(error, results) {
    if(error){
        console.log('ClientConnectionReady Error: ' + error.message);
        client.end();
        return;
    }

    console.log('connection success...\n');

    var content = select.select(client,selectSQLString,function(result){
        console.log(result);
        client.end();
        console.log('Connection closed.\n');
    });

    console.log(content);
});

運行結果：

  connection success...

  select beginning
  select end!

  in select callback function

 { command: 'SELECT',
    rowCount: 4,
    oid: NaN,
    rows:
     [ { id: '1', name: 'aaa', pwd: '111' },
       { id: '2', name: 'bbb', pwd: '222' },
       { id: '3', name: 'ccc', pwd: '333' },
       { id: '4', name: 'ddd', pwd: '444' } ] }
  Connection closed.