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Add reference Markdown: operators/evaluation-query #51

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251 changes: 251 additions & 0 deletions reference/operators/evaluation-query/$expr.md
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---
title: $expr
description: The $expr operator allows the use of aggregation expressions within the query language, enabling complex field comparisons and calculations.
type: operators
category: evaluation-query
---

# $expr

The `$expr` operator allows the use of aggregation expressions within the query language, which enables us to compare fields from the same document, perform calculations, and use aggregation operators in find operations. The `$expr` operator is useful for complex field comparisons that can't be achieved with traditional query operators.

## Syntax

```javascript
{
$expr: { <aggregation expression> }
}
```

## Parameters

| Parameter | Description |
| --- | --- |
| **`<aggregation expression>`** | Any valid aggregation expression that evaluates to a boolean value. The expression includes field comparisons, arithmetic operations, conditional expressions, and other aggregation operators. |

## Examples

Consider this sample document from the stores collection.

```json
{
"_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
"name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
"location": {
"lat": -89.2384,
"lon": -46.4012
},
"staff": {
"totalStaff": {
"fullTime": 8,
"partTime": 20
}
},
"sales": {
"totalSales": 75670,
"salesByCategory": [
{
"categoryName": "Wine Accessories",
"totalSales": 34440
},
{
"categoryName": "Bitters",
"totalSales": 39496
},
{
"categoryName": "Rum",
"totalSales": 1734
}
]
},
"promotionEvents": [
{
"eventName": "Unbeatable Bargain Bash",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 6,
"Day": 23
},
"endDate": {
"Year": 2024,
"Month": 7,
"Day": 2
}
},
"discounts": [
{
"categoryName": "Whiskey",
"discountPercentage": 7
},
{
"categoryName": "Bitters",
"discountPercentage": 15
},
{
"categoryName": "Brandy",
"discountPercentage": 8
},
{
"categoryName": "Sports Drinks",
"discountPercentage": 22
},
{
"categoryName": "Vodka",
"discountPercentage": 19
}
]
},
{
"eventName": "Steal of a Deal Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 9,
"Day": 21
},
"endDate": {
"Year": 2024,
"Month": 9,
"Day": 29
}
},
"discounts": [
{
"categoryName": "Organic Wine",
"discountPercentage": 19
},
{
"categoryName": "White Wine",
"discountPercentage": 20
},
{
"categoryName": "Sparkling Wine",
"discountPercentage": 19
},
{
"categoryName": "Whiskey",
"discountPercentage": 17
},
{
"categoryName": "Vodka",
"discountPercentage": 23
}
]
}
]
}
```

### Example 1: Compare full-time and part-time staff

The example retrieves stores with the number of part-time employees greater than full-time employees.

```javascript
db.stores.find({_id: "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
$expr: {
$gt: ["$staff.employeeCount.partTime", "$staff.employeeCount.fullTime"]
}
})
```

The query compares two fields within the specified (`_id`) document and returns it only if the condition is met (full-time staff count exceeds part-time staff count).

```json
{
"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
"name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
"location": {
"lat": 70.1272,
"lon": 69.7296
},
"staff": {
"contractorCount": 5,
"employeeCount": { "fullTime": 19, "partTime": 20 }
},
"sales": {
"totalSales": 151864,
"salesByCategory": [
{
"categoryName": "Sound Bars",
"totalSales": 2120
},
{
"categoryName": "Home Theater Projectors",
"totalSales": 45004
}
},
"storeOpeningDate": ISODate("2024-09-23T13:45:01.480Z"),
"lastUpdated": ISODate("2025-06-11T11:06:57.922Z"),
"status": "active",
"category": "high-volume",
"priority": 1,
"reviewDate": ISODate("2025-06-11T11:10:50.276Z")
}
]
```

### Example 2: Conditional logic with store location

The example demonstrates the conditional logic usage with `$expr` pulling stores in the southern hemisphere where the staff efficiency ratio (sales per employee) exceeds 2000.

```javascript
db.stores.find({_id: "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
$expr: {
$and: [
{ $gte: ["$location.lat", 70.1272] },
{
$gt: [
{
$divide: [
"$sales.totalSales",
{ $add: ["$staff.employeeCount.fullTime", "$staff.employeeCount.partTime"] }
]
},
2000
]
}
]
}
}).limit(1)
```

This query returns the following result.

```json
[
{
"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
"name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
"location": {
"lat": 70.1272,
"lon": 69.7296
},
"staff": {
"totalStaff": {
"fullTime": 19,
"partTime": 20
}
},
"sales": {
"totalSales": 151864,
"salesByCategory": [
{
"categoryName": "Sound Bars",
"totalSales": 2120
},
{
"categoryName": "Home Theater Projectors",
"totalSales": 45004
}
]
},
"storeOpeningDate": ISODate("2024-09-23T13:45:01.480Z"),
"lastUpdated": ISODate("2025-06-11T11:06:57.922Z"),
"status": "active",
"category": "high-volume",
"priority": 1,
"reviewDate": ISODate("2025-06-11T11:10:50.276Z")
}
]
```
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