$in for matching a set in MongoDB - Step-by-Step Execution

Concept Flow - $in for matching a set

Start Query

↓

Check field value

↓

Is value in $in array?

No→Exclude document

Yes↓

Include document in results

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Repeat for all documents

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Return matched documents

The query checks each document's field value to see if it matches any value in the $in array. If yes, the document is included in the results.

Execution Sample

MongoDB

db.collection.find({ color: { $in: ["red", "blue"] } })

Find documents where the 'color' field is either 'red' or 'blue'.

Execution Table

Step	Document	Field Value	Check $in	Include in Result
1	{"color": "red"}	red	red in ["red", "blue"]? Yes	Yes
2	{"color": "green"}	green	green in ["red", "blue"]? No	No
3	{"color": "blue"}	blue	blue in ["red", "blue"]? Yes	Yes
4	{"color": "yellow"}	yellow	yellow in ["red", "blue"]? No	No
5	End of documents	-	-	-

💡 All documents checked; query returns documents with color 'red' or 'blue'.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Current Document	None	{"color": "red"}	{"color": "green"}	{"color": "blue"}	{"color": "yellow"}	None
Include in Result	No	Yes	No	Yes	No	N/A

Key Moments - 2 Insights

Why does the document with color 'green' not appear in the results?

Because 'green' is not in the $in array ["red", "blue"], so it is excluded as shown in execution_table row 2.

What happens if the $in array is empty?

No document will match since no value can be in an empty set, so all documents are excluded.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result for the document with color 'blue' at step 3?

A Included in results

B Excluded from results

C Query error

D Skipped

💡 Hint

Check the 'Include in Result' column at step 3 in execution_table.

At which step does the condition 'value in $in array' become false?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'No' in the 'Check $in' column in execution_table.

If we add 'yellow' to the $in array, which step's result would change?

A Step 2

B Step 1

C Step 4

D Step 3

💡 Hint

Check step 4 where 'yellow' was previously excluded.

Concept Snapshot

$in operator syntax:
{ field: { $in: [value1, value2, ...] } }
Matches documents where field equals any value in the array.
Useful for filtering by multiple possible values.
Returns all documents with field in the set.
If array empty, no documents match.

Full Transcript

This visual execution shows how MongoDB's $in operator works. The query checks each document's specified field value against the list of values in the $in array. If the value is found in the array, the document is included in the results. For example, documents with color 'red' or 'blue' are included, while others like 'green' or 'yellow' are excluded unless 'yellow' is added to the array. This step-by-step trace helps beginners see how each document is tested and either included or excluded based on the $in condition.