$slice 运算符用于返回数组的子集。 它可用于将数组中的元素数量限制为指定数量，或从数组中的指定位置返回元素。 当处理只有一部分数据用于处理或显示数据的大型数组时，运算符非常有用。
语法
$slice 运算符的语法如下所示。
- 返回数组开头或结尾的元素
{
$slice: [ <array>, <n> ]
}
- 返回数组中指定位置的元素
{
$slice: [ <array>, <position>, <n> ]
}
参数
| 参数 | 说明 |
|---|---|
array |
任何有效的数组表达式。 |
position |
任何有效的整数表达式。 |
n |
任何有效的整数表达式。 |
例子
请考虑商店集合中的这个示例文档。
{
"_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
}
]
}
]
}
示例 1：返回数组中的元素集
此查询检索集合中sales.salesByCategory数组_id: 988d2dd1-2faa-4072-b420-b91b95cbfd60的前三个元素stores。
db.stores.aggregate([{
$match: {
_id: "988d2dd1-2faa-4072-b420-b91b95cbfd60"
}
}, {
$project: {
salesByCategory: {
$slice: ["$sales.salesByCategory", 3]
}
}
}])
此查询返回以下结果。
[
{
"_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
"salesByCategory": [
{
"categoryName": "Towel Racks",
"totalSales": 13237
},
{
"categoryName": "Washcloths",
"totalSales": 44315
},
{
"categoryName": "Face Towels",
"totalSales": 42095
}
]
}
]
示例 2：使用$push切片
此查询用于$push$each向数组添加新元素promotionEvents，并$slice仅保留第一个 N（正切片）或最后 N 个（负切片）元素。 这可确保数组在更新后保留最新的条目。
db.stores.updateOne({
_id: "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5"
}, {
$push: {
promotionEvents: {
$each: [{
eventName: "Black Friday Event",
promotionalDates: {
startDate: {
Year: 2024,
Month: 8,
Day: 1
},
endDate: {
Year: 2024,
Month: 8,
Day: 7
}
},
discounts: [{
categoryName: 'DJ Speakers',
discountPercentage: 25
}]
},
{
eventName: "Mega Discount Days",
promotionalDates: {
startDate: {
Year: 2024,
Month: 5,
Day: 11
},
endDate: {
Year: 2024,
Month: 5,
Day: 18
}
},
discounts: [{
categoryName: "DJ Lights",
discountPercentage: 20
}]
}
],
$slice: -3
}
}
})
查询返回以下结果。
[
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
"location": {
"lat": 60.1441,
"lon": -141.5012
},
"staff": {
"totalStaff": {
"fullTime": 2,
"partTime": 0
}
},
"sales": {
"salesByCategory": [
{
"categoryName": "DJ Headphones",
"totalSales": 35921
},
{
"categoryName": "DJ Cables",
"totalSales": 1000
}
],
"fullSales": 3700
},
"promotionEvents": [
{
"eventName": "Cyber Monday Event",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 8,
"Day": 1
},
"endDate": {
"Year": 2024,
"Month": 8,
"Day": 7
}
},
"discounts": [
{
"categoryName": "DJ Speakers",
"discountPercentage": 25
}
]
},
{
"eventName": "Black Friday Event",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 8,
"Day": 1
},
"endDate": {
"Year": 2024,
"Month": 8,
"Day": 7
}
},
"discounts": [
{
"categoryName": "DJ Speakers",
"discountPercentage": 25
}
]
},
{
"eventName": "Mega Discount Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 5,
"Day": 11
},
"endDate": {
"Year": 2024,
"Month": 5,
"Day": 18
}
},
"discounts": [
{
"categoryName": "DJ Lights",
"discountPercentage": 20
}
]
}
],
"tag": [
"#ShopLocal",
"#NewArrival",
"#FashionStore",
"#SeasonalSale",
"#FreeShipping",
"#MembershipDeals"
]
}
]
示例 3：从数组中提取第一个匹配元素
此查询从“sales.salesByCategory”数组中检索第一个文档。
db.stores.find({
name: "Lakeshore Retail"
}, {
_id: 1,
name: 1,
"sales.salesByCategory": {
$slice: 1
}
} // restricts the fields to be returned
)
此查询返回以下结果。
[
{
"_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
"name": "Lakeshore Retail",
"sales": {
"salesByCategory": [
{
"categoryName": "Towel Racks",
"totalSales": 13237
}
]
}
}
]
示例 4：从数组中提取最后一个元素
此查询从“sales.salesByCategory”数组中检索最后一个文档。
db.stores.find({
name: "Lakeshore Retail"
}, {
_id: 1,
name: 1,
"sales.salesByCategory": {
$slice: -1
}
})
此查询返回以下结果。
[
{
"_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
"name": "Lakeshore Retail",
"sales": {
"salesByCategory": [
{
"categoryName": "Pillow Cases",
"totalSales": 38833
}
]
}
}
]
示例 5：从数组中检索元素范围
此查询从“sales.salesByCategory”数组中检索子集范围。
db.stores.find({
name: "Lakeshore Retail"
}, {
_id: 1,
name: 1,
"sales.salesByCategory": {
$slice: [3, 2]
}
})
此查询返回以下结果。
[
{
"_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
"name": "Lakeshore Retail",
"sales": {
"salesByCategory": [
{
"categoryName": "Toothbrush Holders",
"totalSales": 47912
},
{
"categoryName": "Hybrid Mattresses",
"totalSales": 48660
}
]
}
}
]
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