How to convert this sql query to mongodb - sql

Question

Considering this query written in sql server how would I efficiently convert it to mongodb:
select * from thetable where column1 = column2 * 2

Answer 1

You can use below aggregation.
You project a new field comp to calculate the expression value followed by $match to keep the docs with eq(0) value and $project with exclusion to drop comp field.
db.collection.aggregate([
{ $addFields: {"comp": {$cmp: ["$column1", {$multiply: [ 2, "$column2" ]} ]}}},
{ $match: {"comp":0}},
{ $project:{"comp":0}}
])

Answer 2

If you want to run your query in mongo Shell,
try below code,
db.thetable .find({}).forEach(function(tt){
var ttcol2 = tt.column2 * 2
var comapreCurrent = db.thetable.findOne({_id : tt._id,column1 : ttcol2});
if(comapreCurrent){
printjson(comapreCurrent);
}
});

Answer 3

I liked the answer posted by #Veeram but it would also be possible to achieve this using $project and $match pipeline operation.
This is just for understanding the flow
Assume we have the below 2 documents stored in a math collection
Mongo Documents
{
"_id" : ObjectId("58a055b52f67a312c3993553"),
"num1" : 2,
"num2" : 4
}
{
"_id" : ObjectId("58a055be2f67a312c3993555"),
"num1" : 2,
"num2" : 6
}
Now we need to find if num1 = 2 times of num2 (In our case the document with _id ObjectId("58a055b52f67a312c3993553") will be matching this condition)
Query:
db.math.aggregate([
{
"$project": {
"num2": {
"$multiply": ["$num2",1]
},
"total": {
"$multiply": ["$num1",2]
},
"doc": "$$ROOT"
}
},
{
"$project": {
"areEqual": {"$eq": ["$num2","$total"]
},
doc: 1
}
},
{
"$match": {
"areEqual": true
}
},
{
"$project": {
"_id": 1,
"num1": "$doc.num1",
"num2": "$doc.num2"
}
}
])
Pipeline operation steps:-
The 1st pipeline operation $project calculates the total
The 2nd pipeline operation $project is used to check if the total
matches the num2. This is needed as we cannot use the comparision
operation of num2 with total in the $match pipeline operation
The 3rd pipeline operation matches if areEqual is true
The 4th pipeline operation $project is just used for projecting the fields
Note:-
In the 1st pipeline operation I have multiplied num2 with 1 as num1 and num2 are stored as integers and $multiply returns double value. So incase I do not use $mutiply for num2, then it tries to match 4 equals 4.0 which will not match the document.

Answer 4

Certainly no need for multiple pipeline stages when a single $redact pipeline will suffice as it beautifully incorporates the functionality of $project and $match pipeline steps. Consider running the following pipeline for an efficient query:
db.collection.aggregate([
{
"$redact": {
"$cond": [
{
"$eq": [
"$column1",
{ "$multiply": ["$column2", 2] }
]
},
"$$KEEP",
"$$PRUNE"
]
}
}
])
In the above, $redact will return all documents that match the condition using $$KEEP and discards those that don't match using the $$PRUNE system variable.