问题
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 !
回答1:
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"
}
}
])
来源：https://stackoverflow.com/questions/46958465/how-to-do-left-join-in-db-mongo