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Add reference Markdown: operators/element-query #50

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315 changes: 315 additions & 0 deletions reference/operators/element-query/$exists.md
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---
title: $exists
description: The $exists operator retrieves documents that contain the specified field in their document structure.
type: operators
category: element-query
---

# $exists

The $exists operator retrieves documents that contain the specified field. The $exists operator returns a value of true for documents that contain the specified field, even if the value of the field is null. The $exists operator returns a value of fall for documents that do not contain the specified field in their document structure.

## Syntax

```javascript
{
<field>: { $exists: <true or false> }
}
```

## Parameters

| Parameter | Description |
| --- | --- |
| **`field`** | The field to check for existence. |
| **`true or false`** | `true` for documents that contain the field (including null values), `false` for documents that do not contain the field. |

## 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: Find stores with promotion events

To find any two stores with promotion events, run a query using the $exists operator on the promotionEvents array. Then, project only the ID and promotionEvents fields and limit the results to two documents from the result set.

```javascript
db.stores.find({
"promotionEvents": {
$exists: true
}
}, {
"_id": 1,
"promotionEvents": {
$slice: 1
}
}).limit(2)
```

This query returns the following results:

```json
[
{
"_id": "a715ab0f-4c6e-4e9d-a812-f2fab11ce0b6",
"name": "Lakeshore Retail | Holiday Supply Hub - Marvinfort",
"location": {
"lat": -74.0427,
"lon": 160.8154
},
"staff": {
"employeeCount": {
"fullTime": 9,
"partTime": 18
}
},
"sales": {
"salesByCategory": [
{
"categoryName": "Stockings",
"totalSales": 25731
}
],
"revenue": 25731
},
"promotionEvents": [
{
"eventName": "Mega Savings Extravaganza",
"promotionalDates": {
"startDate": {
"Year": 2023,
"Month": 6,
"Day": 29
},
"endDate": {
"Year": 2023,
"Month": 7,
"Day": 7
}
},
"discounts": [
{
"categoryName": "Stockings",
"discountPercentage": 16
},
{
"categoryName": "Tree Ornaments",
"discountPercentage": 8
}
]
}
],
"company": "Lakeshore Retail",
"city": "Marvinfort",
"storeOpeningDate": "2024-10-01T18:24:02.586Z",
"lastUpdated": "2024-10-02T18:24:02.000Z"
},
{
"_id": "923d2228-6a28-4856-ac9d-77c39eaf1800",
"name": "Lakeshore Retail | Home Decor Hub - Franciscoton",
"location": {
"lat": 61.3945,
"lon": -3.6196
},
"staff": {
"employeeCount": {
"fullTime": 7,
"partTime": 6
}
},
"sales": {
"salesByCategory": [
{
"categoryName": "Lamps",
"totalSales": 19880
},
{
"categoryName": "Rugs",
"totalSales": 20055
}
],
"revenue": 39935
},
"promotionEvents": [
{
"eventName": "Unbeatable Markdown Mania",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 3,
"Day": 25
},
"endDate": {
"Year": 2024,
"Month": 4,
"Day": 1
}
},
"discounts": [
{
"categoryName": "Vases",
"discountPercentage": 8
},
{
"categoryName": "Lamps",
"discountPercentage": 5
}
]
}
],
"company": "Lakeshore Retail",
"city": "Franciscoton",
"lastUpdated": "2024-12-02T12:01:46.000Z",
"storeOpeningDate": "2024-09-03T07:21:46.045Z"
}
]
```

### Example 2: Confirm the presence of a nested field

To retrieve any two stores with full time employees, run a query using the $exists operator on the nested staff.employeeCount.fullTime field. Then, project only the name and ID fields and limit the results to two documents from the result set.

```javascript
db.stores.find({
"staff.employeeCount.fullTime": {
$exists: true
}
}, {
"_id": 1,
"staff.employeeCount": 1
}).limit(2)
```

This query returns the following results:

```json
[
{
"_id": "a715ab0f-4c6e-4e9d-a812-f2fab11ce0b6",
"staff": {
"employeeCount": {
"fullTime": 9,
"partTime": 18
}
}
},
{
"_id": "923d2228-6a28-4856-ac9d-77c39eaf1800",
"staff": {
"employeeCount": {
"fullTime": 7,
"partTime": 6
}
}
}
]
```
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