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

Recall & Review

beginner

What does the $min update operator do in MongoDB?

The $min operator updates the value of a field only if the specified value is less than the current value in the document.

beginner

How does the $max update operator work in MongoDB?

The $max operator updates the value of a field only if the specified value is greater than the current value in the document.

beginner

If a document field has the value 10, what happens when you use $min: 15 to update it?

Nothing changes because 15 is not less than the current value 10. $min only updates if the new value is smaller.

beginner

If a document field has the value 20, what happens when you use $max: 15 to update it?

Nothing changes because 15 is not greater than the current value 20. $max only updates if the new value is larger.

intermediate

Can $min and $max operators be used together in a single update?

Yes, you can use both $min and $max in the same update to conditionally update different fields based on their values.

What will { $min: { score: 50 } } do if the current score is 40?

A Update score to 50

B Keep score as 40

C Remove the score field

D Set score to null

$min updates only if the new value is less than the current. Since 50 > 40, it keeps 40.

Which operator updates a field only if the new value is greater than the current value?

A $min

B $inc

C $set

D $max

$max updates the field only if the new value is greater than the existing value.

If you want to ensure a field never goes below 10, which operator would you use?

A $min with value 10

B $set with value 10

C $max with value 10

D $inc with value 10

Using $max: 10 ensures the field is updated only if the new value is greater than current, so it never goes below 10.

Can $min operator increase the value of a field?

A No, only decreases or keeps the value

B Yes, always

C Yes, but only for strings

D No, it removes the field

$min only updates if the new value is less than the current value, so it never increases the value.

What happens if the field does not exist and you use $max: 5 in an update?

A The field is created with value 5

B The update fails

C The field remains missing

D The field is created with value null

If the field is missing, $max creates it with the specified value.

Explain how the $min and $max update operators work in MongoDB with an example.

Think about when the field value changes or stays the same.

Describe a real-life scenario where using $min or $max operators would be helpful.

Consider tracking scores, prices, or limits.

Practice

1. What does the $min update operator do in MongoDB?

easy

A. Updates the field only if the new value is smaller than the current value.

B. Updates the field only if the new value is larger than the current value.

C. Always updates the field with the new value.

D. Deletes the field if the new value is smaller.

Solution

1. Step 1: Understand the purpose of $min

   The $min operator updates a field only if the new value is smaller than the current value stored in that field.

2. Step 2: Compare with other options

   $max updates if the new value is larger, and other options do not match the behavior of $min.

3. Final Answer:

   Updates the field only if the new value is smaller than the current value. -> Option A

4. Quick Check:

   $min updates only if smaller [OK]

Hint: Remember: $min means update if new value is smaller [OK]

Common Mistakes:

• Confusing $min with $max behavior
• Thinking $min always updates the field
• Assuming $min deletes fields

2. Which of the following is the correct syntax to use $max to update the field score to 90 in MongoDB?

easy

A. { $max: [ score: 90 ] }

B. { $max: score: 90 }

C. { $max: { score: 90 } }

D. { $max: { 'score' => 90 } }

Solution

1. Step 1: Recall correct MongoDB update syntax

   MongoDB update operators use the format: { $operator: { field: value } }. So $max must be followed by an object with field and value.

2. Step 2: Check each option

   { $max: { score: 90 } } matches the correct syntax. The options missing braces around the field-value pair or using brackets [] have invalid syntax. { $max: { 'score' => 90 } } uses '=>' which is not valid in MongoDB queries.

3. Final Answer:

   { $max: { score: 90 } } -> Option C

4. Quick Check:

   Correct MongoDB update syntax = { $max: { score: 90 } } [OK]

Hint: Use braces { } around field-value pairs in update operators [OK]

Common Mistakes:

• Using brackets [] instead of braces {}
• Missing braces around field-value pairs
• Using arrow syntax '=>' instead of colon ':'

3. Given a document { _id: 1, highScore: 80 }, what will be the document after applying db.collection.updateOne({_id:1}, { $max: { highScore: 75 } })?

medium

A. { _id: 1, highScore: 75 }

B. { _id: 1, highScore: 155 }

C. No document updated

D. { _id: 1, highScore: 80 }

Solution

1. Step 1: Understand $max behavior on existing value

   The $max operator updates the field only if the new value is greater than the current value. Here, current highScore is 80, new value is 75.

2. Step 2: Compare values and decide update

   Since 75 is less than 80, the field will NOT be updated. The document remains unchanged.

3. Final Answer:

   { _id: 1, highScore: 80 } -> Option D

4. Quick Check:

   $max updates only if new value is larger [OK]

Hint: If new value < current, $max does not update [OK]

Common Mistakes:

• Assuming $max always updates the field
• Confusing $max with $min behavior
• Thinking update fails if condition not met

4. You run this update: db.collection.updateOne({_id: 2}, { $min: { age: 30 } }) but it does not update the document. What could be the reason?

medium

A. The update syntax is incorrect.

B. The current age in the document is less than 30.

C. The document with _id: 2 does not exist.

D. The $min operator requires an array value.

Solution

1. Step 1: Understand $min update condition

   $min updates the field only if the new value is smaller than the current value.

2. Step 2: Analyze why update might not happen

   If the current age is already less than 30, the update will not change the document. Other options are incorrect because $min does not require arrays, the syntax is correct, and the question states the update runs but no change occurs.

3. Final Answer:

   The current age in the document is less than 30. -> Option B

4. Quick Check:

   $min updates only if new value smaller [OK]

Hint: No update if current value is smaller than $min value [OK]

Common Mistakes:

• Thinking $min requires array values
• Assuming syntax error without checking
• Ignoring existing document values

5. You want to keep track of the lowest and highest temperatures recorded in a MongoDB document with fields minTemp and maxTemp. Which update operation correctly updates both fields when a new temperature temp is 15?

hard

A. { $min: { minTemp: 15 }, $max: { maxTemp: 15 } }

B. { $max: { minTemp: 15 }, $min: { maxTemp: 15 } }

C. { $min: { maxTemp: 15 }, $max: { minTemp: 15 } }

D. { $set: { minTemp: 15, maxTemp: 15 } }

Solution

1. Step 1: Understand the goal for minTemp and maxTemp

   We want minTemp to update only if the new temperature is lower, so use $min. For maxTemp, update only if new temperature is higher, so use $max.

2. Step 2: Check each option for correct operator usage

   { $min: { minTemp: 15 }, $max: { maxTemp: 15 } } correctly uses $min for minTemp and $max for maxTemp. { $max: { minTemp: 15 }, $min: { maxTemp: 15 } } and { $min: { maxTemp: 15 }, $max: { minTemp: 15 } } swap operators incorrectly. { $set: { minTemp: 15, maxTemp: 15 } } always sets both fields to 15, ignoring previous values.

3. Final Answer:

   { $min: { minTemp: 15 }, $max: { maxTemp: 15 } } -> Option A

4. Quick Check:

   Use $min for minTemp and $max for maxTemp [OK]

Hint: Use $min for minimum and $max for maximum fields [OK]

Common Mistakes:

• Swapping $min and $max operators
• Using $set which overwrites values blindly
• Updating wrong fields with wrong operators