$min and $max accumulators in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $min accumulator do in MongoDB aggregation?

The $min accumulator finds the smallest value for a specified field across grouped documents.

beginner

What is the purpose of the $max accumulator in MongoDB?

The $max accumulator returns the largest value for a specified field from grouped documents.

intermediate

How do $min and $max behave when used in a $group stage?

They compute the minimum and maximum values respectively for each group of documents defined by the _id field in the $group stage.

intermediate

Can $min and $max be used on fields with different data types?

They work best on comparable data types like numbers or dates. Mixing incompatible types can lead to unexpected results or errors.

beginner

Example: How to find the minimum and maximum price of products in each category using MongoDB aggregation?

Use $group with _id as category, and $min: "$price" and $max: "$price" to get min and max prices per category.

What does the $min accumulator return in a MongoDB aggregation?

A The smallest value of a field in grouped documents

B The largest value of a field in grouped documents

C The average value of a field

D The total count of documents

$min finds the smallest value for a specified field within each group.

Which accumulator would you use to find the highest score in a group of documents?

A <code>$max</code>

B <code>$min</code>

C <code>$avg</code>

D <code>$sum</code>

$max returns the largest value in the group.

In which aggregation stage are $min and $max typically used?

A <code>$sort</code>

B <code>$project</code>

C <code>$match</code>

D <code>$group</code>

$min and $max are used in $group to aggregate values per group.

What happens if you use $min on a field with mixed data types?

A It always returns the smallest number

B It may return unexpected results or errors

C It converts all values to strings

D It ignores non-numeric values

Using $min on mixed types can cause errors or unexpected results because values are not comparable.

How do you specify the field to find the minimum value of in $min?

A Use the field name in an array

B Use the field name as a string without $

C Use the field name prefixed with $ like "$fieldName"

D Use the field name in a nested object

You specify the field with a $ prefix, for example "$price".

Explain how $min and $max accumulators work in MongoDB aggregation.

Think about grouping documents and finding extreme values.

Describe a real-life example where you would use $min and $max in a database query.

Consider a store with products or a class with student scores.

Practice

1. What do the $min and $max accumulators do in a MongoDB aggregation pipeline?

easy

A. $min and $max sort documents in ascending and descending order respectively.

B. $min and $max both find the average value in a group.

C. $min finds the first document and $max finds the last document in a group.

D. $min finds the smallest value and $max finds the largest value in a group.

Solution

1. Step 1: Understand the purpose of $min and $max

   $min returns the smallest value from the grouped documents, while $max returns the largest value.

2. Step 2: Compare with other options

   The other options describe different operations like average, first/last document, or sorting, which are not what $min and $max do.

3. Final Answer:

   $min finds the smallest value and $max finds the largest value in a group. -> Option D

4. Quick Check:

   $min = smallest, $max = largest [OK]

Hint: Remember: min = smallest, max = largest value [OK]

Common Mistakes:

• Confusing $min and $max with average functions
• Thinking they return documents instead of values
• Mixing accumulators with sorting operators

2. Which of the following is the correct syntax to use $max in a $group stage to find the highest score from a field named score?

easy

A. { $group: { _id: null, maxScore: { $max: "$score" } } }

B. { $group: { _id: null, maxScore: { max: "$score" } } }

C. { $group: { _id: null, maxScore: { $max: score } } }

D. { $group: { _id: null, maxScore: { $max: "score" } } }

Solution

1. Step 1: Check the correct use of $max accumulator

   The correct syntax requires $max as the operator and the field name as a string with a $ prefix inside quotes: "$score".

2. Step 2: Identify syntax errors in other options

   { $group: { _id: null, maxScore: { max: "$score" } } } misses the $ before max, { $group: { _id: null, maxScore: { $max: score } } } lacks quotes around field name, and { $group: { _id: null, maxScore: { $max: "score" } } } uses quotes without $ prefix, all invalid.

3. Final Answer:

   { $group: { _id: null, maxScore: { $max: "$score" } } } -> Option A

4. Quick Check:

   Use $max: "$field" syntax [OK]

Hint: Always prefix field names with $ and use quotes inside accumulators [OK]

Common Mistakes:

• Omitting $ before accumulator name
• Not quoting the field name with $
• Using field name without $ or quotes

3. Given the collection documents:

{ "name": "Alice", "score": 85 }, { "name": "Bob", "score": 92 }, { "name": "Charlie", "score": 78 }

What will be the output of this aggregation?

[{ $group: { _id: null, minScore: { $min: "$score" }, maxScore: { $max: "$score" } } }]

medium

A. { "_id": null, "minScore": 85, "maxScore": 85 }

B. { "_id": null, "minScore": 78, "maxScore": 92 }

C. { "_id": null, "minScore": 78, "maxScore": 78 }

D. { "_id": null, "minScore": 92, "maxScore": 85 }

Solution

1. Step 1: Identify minimum and maximum scores from documents

   The scores are 85, 92, and 78. The smallest is 78 and the largest is 92.

2. Step 2: Understand the aggregation output

   The $group stage groups all documents (_id: null) and calculates minScore as 78 and maxScore as 92.

3. Final Answer:

   { "_id": null, "minScore": 78, "maxScore": 92 } -> Option B

4. Quick Check:

   Min = 78, Max = 92 [OK]

Hint: Scan values, pick smallest for $min and largest for $max [OK]

Common Mistakes:

• Mixing up min and max values
• Assuming $min or $max returns documents instead of values
• Forgetting to use $ before field names

4. Consider this aggregation pipeline:

[{ $group: { _id: "$category", minPrice: { $min: price }, maxPrice: { $max: "$price" } } }]

What is the error in this pipeline?

medium

A. Field name 'price' in $min is missing quotes and $ prefix.

B. Using $max and $min together is not allowed.

C. The _id field cannot be a field name.

D. The pipeline is correct and has no errors.

Solution

1. Step 1: Check syntax of $min and $max usage

   The $min accumulator uses price without quotes and $ prefix, which is incorrect. It should be "$price".

2. Step 2: Verify other parts of the pipeline

   $max is correctly used with "$price". Grouping by "$category" is valid. Using both accumulators together is allowed.

3. Final Answer:

   Field name 'price' in $min is missing quotes and $ prefix. -> Option A

4. Quick Check:

   Field names need "$" prefix inside accumulators [OK]

Hint: Always quote and prefix field names with $ inside accumulators [OK]

Common Mistakes:

• Omitting $ before field names inside accumulators
• Thinking $min and $max cannot be used together
• Misunderstanding _id field usage in $group

5. You have a collection of sales documents with fields region and amount. You want to find the smallest and largest sale amounts per region, but only for regions where the smallest sale is above 100. Which aggregation pipeline correctly achieves this?

hard

A. [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" }, filter: { $gt: ["$minSale", 100] } } }]

B. [{ $match: { amount: { $gt: 100 } } }, { $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }]

C. [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }, { $match: { minSale: { $gt: 100 } } }]

D. [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }, { $match: { maxSale: { $gt: 100 } } }]

Solution

1. Step 1: Group by region and calculate min and max sales

   The $group stage groups documents by region and calculates minSale and maxSale using $min and $max on amount.

2. Step 2: Filter groups where minSale is greater than 100

   The $match stage after grouping filters groups where minSale is greater than 100, exactly as required.

3. Final Answer:

   [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }, { $match: { minSale: { $gt: 100 } } }] -> Option C

4. Quick Check:

   Group first, then filter on aggregated minSale [OK]

Hint: Filter after grouping to use aggregated fields in conditions [OK]

Common Mistakes:

• Filtering before grouping, which filters documents not groups
• Trying to filter inside $group stage incorrectly
• Filtering on maxSale instead of minSale