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.