Mongodb：如何做像mysql的`groupby`这样的工作？ [重复]

Question

This question already has an answer here:

• Mongodb Aggregation Framework | Group over multiple values? 1 answer

I am using MongoDB PHP Library to malipulate mongodb data.
docs: https://docs.mongodb.com/php-library/master/tutorial/collation/#aggregation

How to do a work in mongodb like groupby of mysql?

for example,there is a collection:

    $collection=[
        ['id'=>'xxx','county'=>'Montgomery','state'=>'Alabama','abbr'=>'AL'],
        ['id'=>'xxx','county'=>'Calhoun','state'=>'Alabama','abbr'=>'AL'],
        ['id'=>'xxx','county'=>'Alameda','state'=>'California','abbr'=>'CA'],
        ['id'=>'xxx','county'=>'Butte','state'=>'California','abbr'=>'CA'],
        ['id'=>'xxx','county'=>'Baker','state'=>'Georgia','abbr'=>'GA'],
        ['id'=>'xxx','county'=>'Clarke','state'=>'Georgia','abbr'=>'GA'],
    ];

I want to get fields state and abbr,and save them into a new collection,the result is like this:

$newCollection=[
    ['id'=>'xxx','state'=>'Alabama','abbr'=>'AL'],
    ['id'=>'xxx','state'=>'California','abbr'=>'CA'],
    ['id'=>'xxx','state'=>'Georgia','abbr'=>'GA'],
];

How to do it?

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Answer 1

MongoDB equivalent of the group by statement is aggregation framework (https://docs.mongodb.com/manual/reference/method/db.collection.aggregate/). Though for your requirement, if you want a new collection, you will have to use mapReduce or MongoDB views.