node.js 操作postgresql数据库（2）

在上一篇 node.js操作postgresql数据库中，还有很多需要进行完善的地方，而这篇文章主要完善以下几点：
1）将查询的结果返还给客户端
2）访问数据库的正常退出
3）返回值的问题


1）将查询的结果返还给客户端：
   一般的查询都需要将结果展示出来，否则查询将显得没有意义。在node.js操作postgresql数据库中，如何将查询的数据返回回来呢？

  (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;        }         //在执行完查询以后，结果集被存放在results中,你可以使用console.log(results)打印出来看看       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);    });});

  //采用回调函数的形式来获取select.js文件中的查询结果

  执行结果为：

     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) 访问数据库的正常退出：

      由于node.js的特性，若直接在调用select函数之后就关闭连接，结果可能就和我們预想的不一样了：

  select.js不变,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');});

   运行结果：

              connection success...

                 Connection closed.

    可以看出，并没有将查询的结果返回回来，连接就关闭了。这是因为node.js执行到查询的时候，采用非阻塞的方式，直接跳过执行后面的语句，当查询执行完毕和调用相应的回调函数。

正确的处理方式为：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');    });});

运行结果：

 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)  返回值的问题:大多数情况下，函数都有返回值

   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);});

运行结果：

  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.