In MongoDB, there isn't a direct equivalent of the EXCEPT operator found in SQL databases. However, you can achieve similar results using the aggregation framework, specifically using the $lookup stage to perform a left outer join between two collections. Here's how you can do it with an example:
Suppose you have two collections: employees and former_employees, both with similar structures, and you want to retrieve all documents from the employees collection that are not present in the former_employees collection.
Let's assume the following structures for the collections:
// employees collection
{
"_id": ObjectId("..."),
"employee_id": 1,
"first_name": "John",
"last_name": "Doe"
}
// former_employees collection
{
"_id": ObjectId("..."),
"employee_id": 2,
"first_name": "Jane",
"last_name": "Smith"
}
You can use the aggregation framework to achieve this:
db.employees.aggregate([
{
$lookup: {
from: "former_employees",
localField: "employee_id",
foreignField: "employee_id",
as: "former_employee"
}
},
{
$match: {
former_employee: { $eq: [] } // Filter out documents where former_employee array is empty
}
},
{
$project: {
former_employee: 0 // Remove the former_employee field from the output
}
}
])
Explanation of each stage:
1. $lookup: This stage performs a left outer join between the employees collection and the former_employees collection based on the employee_id field. It adds an array field former_employee to each document containing matching documents from the former_employees collection.
2. $match: This stage filters out documents where the former_employee array is empty, meaning there are no matches in the former_employees collection.
3. $project: This stage removes the former_employee field from the output documents to keep the result clean.
This aggregation pipeline will effectively retrieve all documents from the employees collection that are not present in the former_employees collection, simulating the behavior of the EXCEPT operator in SQL.
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