Date expressions ($year, $month, $dayOfMonth) in MongoDB

Introduction

Date expressions help you get parts of a date like the year, month, or day from a date stored in your database. This makes it easy to organize or filter data by date.

You want to find all records from a specific year, like sales in 2023.

You need to group data by month to see monthly trends.

You want to filter events that happened on a certain day of the month.

You are creating reports that show data by year, month, or day.

You want to sort data based on the year or month part of a date.

Syntax

MongoDB

{ $year: <dateExpression> }
{ $month: <dateExpression> }
{ $dayOfMonth: <dateExpression> }

<dateExpression> is the date field or expression you want to extract parts from.

These expressions return numbers: year (e.g., 2024), month (1-12), day of month (1-31).

Examples

Extracts the year from the orderDate field.

MongoDB

{ $year: "$orderDate" }

Gets the month number from the createdAt date.

MongoDB

{ $month: "$createdAt" }

Finds the day of the month from the eventDate field.

MongoDB

{ $dayOfMonth: "$eventDate" }

Sample Program

This query takes the orderDate from each order and creates new fields showing the year, month, and day of that date. It also keeps the item field to see what was ordered.

MongoDB

db.orders.aggregate([
  {
    $project: {
      year: { $year: "$orderDate" },
      month: { $month: "$orderDate" },
      day: { $dayOfMonth: "$orderDate" },
      item: 1
    }
  }
])

Important Notes

If the date field is missing or null, these expressions return null.

Months are numbered 1 for January through 12 for December.

Use these expressions inside aggregation pipelines like $project or $group.

Summary

Use $year, $month, and $dayOfMonth to get parts of a date.

They help organize and filter data by date parts easily.

These expressions work inside MongoDB aggregation pipelines.

Practice

1. What does the MongoDB expression {$year: "$dateField"} return when used in an aggregation pipeline?

easy

A. The month number of the date

B. The year part of the date stored in dateField

C. The full date as a string

D. The day of the week of the date

Solution

1. Step 1: Understand the $year operator

   The $year expression extracts the year component from a date value.

2. Step 2: Apply to the field dateField

   Using {$year: "$dateField"} returns the year number from the date stored in dateField.

3. Final Answer:

   The year part of the date stored in dateField -> Option B

4. Quick Check:

   $year extracts year [OK]

Hint: Remember $year extracts only the year number from a date [OK]

Common Mistakes:

• Confusing $year with $month or $dayOfMonth
• Expecting a full date string instead of just the year
• Using $year outside aggregation pipeline

2. Which of the following is the correct syntax to extract the month from a date field named createdAt in a MongoDB aggregation pipeline?

easy

A. { $month: "$createdAt" }

B. { $month: createdAt }

C. { month: "$createdAt" }

D. { $month: "createdAt" }

Solution

1. Step 1: Identify correct field reference syntax

   In MongoDB aggregation, field names must be referenced as strings with a $ prefix, like "$createdAt".

2. Step 2: Confirm correct operator usage

   The operator to extract month is $month, so the correct syntax is { $month: "$createdAt" }.

3. Final Answer:

   { $month: "$createdAt" } -> Option A

4. Quick Check:

   Field names need "$" prefix in aggregation [OK]

Hint: Use "$fieldName" to reference fields inside aggregation [OK]

Common Mistakes:

• Omitting quotes and $ prefix around field name
• Using operator without $ sign
• Passing field name as plain string without $

3. Given the following aggregation stage, what will be the output for a document with dateField value of ISODate("2023-07-15T10:00:00Z")?

{ $project: { year: { $year: "$dateField" }, month: { $month: "$dateField" }, day: { $dayOfMonth: "$dateField" } } }

medium

A. { year: 2023, month: 7, day: 14 }

B. { year: 2023, month: 6, day: 15 }

C. { year: 2023, month: 7, day: 15 }

D. { year: 2022, month: 7, day: 15 }

Solution

1. Step 1: Extract year, month, and day from the date

   The date is July 15, 2023. So, year = 2023, month = 7 (July), day = 15.

2. Step 2: Confirm output matches extracted values

   The output document will have year: 2023, month: 7, day: 15 as projected.

3. Final Answer:

   { year: 2023, month: 7, day: 15 } -> Option C

4. Quick Check:

   Date parts match output [OK]

Hint: Remember months are 1-based in MongoDB date expressions [OK]

Common Mistakes:

• Assuming months start at 0 instead of 1
• Confusing dayOfMonth with dayOfWeek
• Misreading the date value

4. You wrote this aggregation stage to get the day of the month from orderDate:

{ $project: { day: { $dayOfMonth: orderDate } } }

But it throws an error. What is the mistake?

medium

A. Missing quotes and $ before field name in $dayOfMonth

B. Using $project instead of $match

C. Field orderDate is not a date type

D. Incorrect operator; should use $day instead

Solution

1. Step 1: Check field reference syntax inside $dayOfMonth

   Field names must be referenced as strings with a $ prefix, so it should be "$orderDate".

2. Step 2: Identify the cause of error

   Using orderDate without quotes and $ causes a syntax error in aggregation.

3. Final Answer:

   Missing quotes and $ before field name in $dayOfMonth -> Option A

4. Quick Check:

   Field names need "$" prefix in aggregation [OK]

Hint: Always use "$fieldName" inside aggregation expressions [OK]

Common Mistakes:

• Forgetting $ and quotes around field names
• Assuming $day is a valid operator
• Confusing aggregation stages

5. You want to group sales documents by the year and month of their saleDate field. Which aggregation pipeline stage correctly groups by year and month using date expressions?

hard

A. { $group: { _id: { year: { $year: "$saleDate" }, month: "$month" }, totalSales: { $sum: "$amount" } } }

B. { $group: { _id: { year: "$year", month: "$month" }, totalSales: { $sum: "$amount" } } }

C. { $group: { _id: { year: { $year: saleDate }, month: { $month: saleDate } }, totalSales: { $sum: "$amount" } } }

D. { $group: { _id: { year: { $year: "$saleDate" }, month: { $month: "$saleDate" } }, totalSales: { $sum: "$amount" } } }

Solution

1. Step 1: Use correct field references with $year and $month

   Field names must be referenced as strings with $ prefix, so "$saleDate" is correct.

2. Step 2: Verify grouping keys and sum operator

   Grouping by year and month extracted from saleDate and summing amount is done correctly in { $group: { _id: { year: { $year: "$saleDate" }, month: { $month: "$saleDate" } }, totalSales: { $sum: "$amount" } } }.

3. Final Answer:

   { $group: { _id: { year: { $year: "$saleDate" }, month: { $month: "$saleDate" } }, totalSales: { $sum: "$amount" } } } -> Option D

4. Quick Check:

   Correct field references and operators used [OK]

Hint: Group by {_id: {year: {$year: "$field"}, month: {$month: "$field"}}} [OK]

Common Mistakes:

• Using field names without $ and quotes
• Mixing string field names with expressions
• Incorrectly mixing aggregation operators