mongodb中有一个字段为list类型, 经常有个需求就是要对list内的元素进行计数统计, 所以记录下此种问题的解决方法(如果是用pymongo来测试, 务必要用双引号把操作符包起来如: "$set")
先新建一个数据源例子
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db.getCollection('test').insert([
{
'tags':['a','b','d']
},
{
'tags':['a','d']
},
{
'tags':['b','d']
}
])
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我们期望的结果是能够统计tags标签中的元素个数, 比如类似这样的结果
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/* 1 */
{
"_id" : "d",
"count" : 3.0
}
/* 2 */
{
"_id" : "b",
"count" : 2.0
}
/* 3 */
{
"_id" : "a",
"count" : 2.0
}
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SQL
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db.getCollection('test').aggregate([
{$unwind: "$tags"},
{$group: {_id: "$tags", count: {$sum: 1}}},
]);
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你可能想把"_id"这个key替换成别的什么名字, 比如"name", 只需要给上面的SQL加一条
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db.getCollection('test').aggregate([
{$unwind: "$tags"},
{$group: {_id: "$tags", count: {$sum: 1}}},
{$project: {name: "$_id", count: "$count"}}
])
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于是得到了如下的结果
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/* 1 */
{
"_id" : "d",
"name" : "d",
"count" : 3.0
}
/* 2 */
{
"_id" : "b",
"name" : "b",
"count" : 2.0
}
/* 3 */
{
"_id" : "a",
"name" : "a",
"count" : 2.0
}
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那个"_id"太讨厌, 去掉行不行? 当然行
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db.getCollection('test').aggregate([
{$unwind: "$tags"},
{$group: {_id: "$tags", count: {$sum: 1}}},
{$project: {name: "$_id", count: "$count"}, _id: 0}
])
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所以最终的结果长这样
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/* 1 */
{
"name" : "d",
"count" : 3.0
}
/* 2 */
{
"name" : "b",
"count" : 2.0
}
/* 3 */
{
"name" : "a",
"count" : 2.0
}
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如果还想要更屌的操作, 这几个操作符可以好好查文档看看
$unwind, $group, $project
https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/
https://docs.mongodb.com/manual/reference/operator/aggregation/group/
https://docs.mongodb.com/manual/reference/operator/aggregation/project/