紧接着上篇来，这篇主要讲，mongodb的group功能，做的还是挺强大的，相当对于find()，skip()，distinct()等，用法比较复杂。

测试数据：
复制代码 代码如下:
> db.fruit.find();
{ "_id" : 1, "category" : "fruit", "name" : "apple" }
{ "_id" : 2, "category" : "fruit", "name" : "peach" }
{ "_id" : 3, "category" : "fruit", "name" : "banana" }
{ "_id" : 4, "category" : "veggie", "name" : "corn" }
{ "_id" : 5, "category" : "veggie", "name" : "broccoli" }

1、根据category分组
复制代码 代码如下:
> db.fruit.group(
{
key: { category: 1},
reduce: function(obj, prev) {
prev.items.push(obj.name);
},
initial: { items : [] }
}
);
[
{
"category" : "fruit",
"items" : [
"apple",
"peach",
"banana"
]
},
{
"category" : "veggie",
"items" : [
"corn",
"broccoli"
]
}
]

php代码如下：
复制代码 代码如下:
$keys = array("category" => 1);
$initial = array("items" => array());
$reduce = "function (obj, prev) { prev.items.push(obj.name); }";
$g = $collection->group($keys, $initial, $reduce);

print_r($g); //结果如下。

Array
(
[retval] => Array
(
[0] => Array
(
[category] => fruit
[items] => Array
(
[0] => apple
[1] => peach
[2] => banana
)

)

[1] => Array
(
[category] => veggie
[items] => Array
(
[0] => corn
[1] => broccoli
)

)

)

[count] => 5
[keys] => 2
[ok] => 1
)

2、根据category来分组，并统计count

复制代码 代码如下:
> db.fruit.group(
{
key: { category: 1},
cond: { _id: { $gt: 2 } },
reduce: function(obj, prev) {
prev.items.push(obj.name);
prev.count++;
},
initial: { items : [] ,count:0}
}
);
[
{
"category" : "fruit",
"items" : [
"banana"
],
"count" : 1
},
{
"category" : "veggie",
"items" : [
"corn",
"broccoli"
],
"count" : 2
}
]

php代码如下：

复制代码 代码如下:
$keys = array("category" => 1);
$initial = array("items" => array(),'count'=>0);
$reduce = "function (obj, prev) { " .
"prev.items.push(obj.name); " .
"prev.count++;" .
"}";
$condition = array('condition' => array("_id" => array( '$gt' => 2)));
$g = $collection->group($keys, $initial, $reduce, $condition);

print_r($g); //结果如下。

Array
(
[retval] => Array
(
[0] => Array
(
[category] => fruit
[items] => Array
(
[0] => banana
)

[count] => 1
)

[1] => Array
(
[category] => veggie
[items] => Array
(
[0] => corn
[1] => broccoli
)

[count] => 2
)
)

[count] => 3
[keys] => 2
[ok] => 1
)

3、利用aggregate group功能，也挺强大

复制代码 代码如下:
> db.fruit.aggregate([
{ $match: { _id: {$gt:0} } },
{ $group: { _id: "$category", count: { $sum: 1 } } },
{ $sort: { count: -1 } }
]);
{ "_id" : "fruit", "count" : 3 }
{ "_id" : "veggie", "count" : 2 }

php代码如下：
复制代码 代码如下:
$cond = array(
array(
'$match' => array('_id' => array('$gt' => 0)),
),
array(
'$group' => array(
'_id' => '$category',
'count' => array('$sum' => 1),
),
),
array(
'$sort' => array("count" => -1),
),
);
$result = $collection->aggregate($cond);
print_r($result); //结果如下：

Array
(
[result] => Array
(
[0] => Array
(
[_id] => fruit
[count] => 3
)

[1] => Array
(
[_id] => veggie
[count] => 2
)

)

[ok] => 1
)

mongodb 的select 操作有很多，在这里，只是说了一些常用的功能。