如何在数据库中进行左联接(Mongo) [英] How to do Left Join in DB (Mongo)

问题描述

我是Mongo的新手！请帮助我如何在Mongo中退出加入

I am new to Mongo! Please help me how to do left join in Mongo

Sql语句:

Select * from TableA left Join TableB 
on (TableA.col1 = TableB.col1 AND TableB.col2 = "ABC")

请向我提供等效的Mongo查询！！！

Please provide me the equivalent Mongo Query!!!

先谢谢了！

推荐答案

从Mongo 3.2开始，您可以通过向聚合管道中添加新的$ lookup运算符来进行等效于左外部联接的操作:

As of Mongo 3.2, you can do the equivalent to a left outer join with the new $lookup operator added to the aggregation pipeline: https://docs.mongodb.org/master/reference/operator/aggregation/lookup/#pipe._S_lookup

您的查询将变成这样:

db.TableB.aggregate([
{
  $match:{col2:"ABC"}
},
{
   $lookup:
   {
       from: TableA,
       localField: "col1",
       foreignField: "col1",
       as: "aliasForTable1Collection"
   }
}
])

这篇关于如何在数据库中进行左联接(Mongo)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！