如何在数据库中进行左连接(Mongo) [英] How to do Left Join in DB (Mongo)
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问题描述
I am new to Mongo! Please help me how to do left join in Mongo
Sql Statement :
Select * from TableA left Join TableB
on (TableA.col1 = TableB.col1 AND TableB.col2 = "ABC")
Please provide me the equivalent Mongo Query!!!
Thanks In Advance !
解决方案
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
Your query would become something like this:
db.TableB.aggregate([
{
$match:{col2:"ABC"}
},
{
$lookup:
{
from: "TableA",
localField: "col1",
foreignField: "col1",
as: "aliasForTable1Collection"
}
}
])
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