MongoDB在MySQL中替换GROUP_CONCAT [英] Mongodb replacement for GROUP_CONCAT in mysql

问题描述

亲爱的，

我们已经开始在内部项目中使用mongodb.我遇到了一些问题，需要您的帮助.

Dear All,

We have started using mongodb for our internal project. I am facing some issues and need your help.

这是我的桌子... bookings_issued

Here is my table... books_issued

user_id    book_id
1                 1个
1                 3
2                 5
2                 8

user_id    book_id
1                1
1                3
2                5
2                8

我想使用$ group列出它，结果应该是这样的...
user_id    book_id
1                                     1,3
2                                    5,8

I want to list it using the $group and result should be like this...
user_id    book_id
1                1,3
2                5,8

我们正在使用$ match和$ group.请帮助使用Java api解决相同问题.

We are using $match and $group. Please help to solve the same using java api.

谢谢，

推荐答案

此查询使用user_id作为聚合的_id字段，并将book_id添加到books数组:

This query uses the user_id as _id field for the aggregate and adds the book_id to the books array:

db.books_issued.aggregate( {$group:  { _id: "$user_id", books: {$push: "$book_id"} } } )

java代码如下:

    DBObject groupObj = new BasicDBObject().append("$group", 
            new BasicDBObject().append("_id", "$user_id")
            .append("books", new BasicDBObject("$push", "$book_id")));
    List<DBObject> pipeline = new ArrayList<DBObject>();
    pipeline.add(groupObj);
    AggregationOutput r = books.aggregate(pipeline);

这篇关于MongoDB在MySQL中替换GROUP_CONCAT的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！