Introduction to MongoDB $min Operator

The MongoDB $min operator is used in an aggregation pipeline to find the minimum value of a specified field in a collection and return it. This operator can be used in various query scenarios, such as finding the minimum value or earliest date.

Syntax

The syntax of the $min operator is as follows:

{ $min: <expression> }

Where <expression> is a field or expression used to find the minimum value.

Use Cases

The $min operator is suitable for scenarios that require finding the minimum value of a specified field, such as:

• Finding the earliest date within a certain time range.
• Finding the lowest price of a specific product.
• Finding the minimum age of a person, and so on.

Examples

Here are three examples of the $min operator:

Example 1: Finding the Minimum Value

Assume there is a students collection that contains the name and age of each student. Now we want to find the student with the minimum age.

The aggregation pipeline that uses the $min operator is as follows:

db.students.aggregate([{ $group: { _id: null, minAge: { $min: "$age" } } }])

After running the pipeline, the following result will be returned:

{ "_id": null, "minAge": 18 }

This indicates that the minimum age is 18 years old.

Example 2: Finding the Earliest Date

Assume there is an orders collection that contains the date and amount of each order. Now we want to find the earliest order date.

The aggregation pipeline that uses the $min operator is as follows:

db.orders.aggregate([{ $group: { _id: null, minDate: { $min: "$date" } } }])

After running the pipeline, the following result will be returned:

{ "_id": null, "minDate": ISODate("2022-01-01T00:00:00Z") }

This indicates that the earliest order date is January 1st, 2022.

Example 3: Finding the Lowest Price

Assume there is a products collection that contains the name and price of each product. Now we want to find the cheapest product.

The aggregation pipeline that uses the $min operator is as follows:

db.products.aggregate([{ $group: { _id: null, minPrice: { $min: "$price" } } }])

After running the pipeline, the following result will be returned:

{ "_id": null, "minPrice": 10 }

This indicates that the lowest price is 10 yuan.

Conclusion

The $min operator is a very useful aggregation operator that can find the minimum value of a specified field in a collection and return it in an aggregation pipeline. Through the introduction and examples in this article, you can better understand how to use the $min operator.