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- $substrBytes (aggregation)
$substrBytes (aggregation)¶
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Definition¶
- $substrBytes¶
3.4 新版功能.
Returns the substring of a string. The substring starts with the character at the specified UTF-8 byte index (zero-based) in the string and continues for the number of bytes specified.
$substrBytes has the following operator expression syntax:
{ $substrBytes: [ <string expression>, <byte index>, <byte count> ] }
Field Type Description string expression string The string from which the substring will be extracted. string expression can be any valid expression as long as it resolves to a string. For more information on expressions, see Expressions.
If the argument resolves to a value of null or refers to a field that is missing, $substrBytes returns an empty string.
If the argument does not resolve to a string or null nor refers to a missing field, $substrBytes returns an error.
byte index number Indicates the starting point of the substring. byte index can be any valid expression as long as it resolves to a non-negative integer or number that can be represented as an integer (such as 2.0).
byte index cannot refer to a starting index located in the middle of a multi-byte UTF-8 character.
byte count number Can be any valid expression as long as it resolves to a non-negative integer or number that can be represented as an integer (such as 2.0).
byte count can not result in an ending index that is in the middle of a UTF-8 character.
Behavior¶
The $substrBytes operator uses the indexes of UTF-8 encoded bytes where each code point, or character, may use between one and four bytes to encode.
For example, US-ASCII characters are encoded using one byte. Characters with diacritic markings and additional Latin alphabetical characters (i.e. Latin characters outside of the English alphabet) are encoded using two bytes. Chinese, Japanese and Korean characters typically require three bytes, and other planes of unicode (emoji, mathematical symbols, etc.) require four bytes.
It is important to be mindful of the content in the string expression because providing a byte index or byte count located in the middle of a UTF-8 character will result in an error.
$substrBytes differs from $substrCP in that $substrBytes counts the bytes of each character, whereas $substrCP counts the code points, or characters, regardless of how many bytes a character uses.
| Example | Results |
|---|---|
| { $substrBytes: [ "abcde", 1, 2 ] } | "bc" |
| { $substrBytes: [ "Hello World!", 6, 5 ] } | "World" |
| { $substrBytes: [ "cafétéria", 0, 5 ] } | "café" |
| { $substrBytes: [ "cafétéria", 5, 4 ] } | "tér" |
| { $substrBytes: [ "cafétéria", 7, 3 ] } | "Error: Invalid range, starting index is a UTF-8 continuation byte." |
| { $substrBytes: [ "cafétéria", 3, 1 ] } | "Error: Invalid range, ending index is in the middle of a UTF-8 character." |
Example¶
Single-Byte Character Set¶
Consider an inventory collection with the following documents:
{ "_id" : 1, "item" : "ABC1", quarter: "13Q1", "description" : "product 1" }
{ "_id" : 2, "item" : "ABC2", quarter: "13Q4", "description" : "product 2" }
{ "_id" : 3, "item" : "XYZ1", quarter: "14Q2", "description" : null }
The following operation uses the $substrBytes operator separate the quarter value (containing only single byte US-ASCII characters) into a yearSubstring and a quarterSubstring. The quarterSubstring field represents the rest of the string from the specified byte index following the yearSubstring. It is calculated by subtracting the byte index from the length of the string using $strLenBytes.
db.inventory.aggregate(
[
{
$project: {
item: 1,
yearSubstring: { $substrBytes: [ "$quarter", 0, 2 ] },
quarterSubtring: {
$substrBytes: [
"$quarter", 2, { $subtract: [ { $strLenBytes: "$quarter" }, 2 ] }
]
}
}
}
]
)
The operation returns the following results:
{ "_id" : 1, "item" : "ABC1", "yearSubstring" : "13", "quarterSubtring" : "Q1" }
{ "_id" : 2, "item" : "ABC2", "yearSubstring" : "13", "quarterSubtring" : "Q4" }
{ "_id" : 3, "item" : "XYZ1", "yearSubstring" : "14", "quarterSubtring" : "Q2" }
Single-Byte and Multibyte Character Set¶
A collection named food contains the following documents:
{ "_id" : 1, "name" : "apple" }
{ "_id" : 2, "name" : "banana" }
{ "_id" : 3, "name" : "éclair" }
{ "_id" : 4, "name" : "hamburger" }
{ "_id" : 5, "name" : "jalapeño" }
{ "_id" : 6, "name" : "pizza" }
{ "_id" : 7, "name" : "tacos" }
{ "_id" : 8, "name" : "寿司sushi" }
The following operation uses the $substrBytes operator to create a three byte menuCode from the name value:
db.food.aggregate(
[
{
$project: {
"name": 1,
"menuCode": { $substrBytes: [ "$name", 0, 3 ] }
}
}
]
)
The operation returns the following results:
{ "_id" : 1, "name" : "apple", "menuCode" : "app" }
{ "_id" : 2, "name" : "banana", "menuCode" : "ban" }
{ "_id" : 3, "name" : "éclair", "menuCode" : "éc" }
{ "_id" : 4, "name" : "hamburger", "menuCode" : "ham" }
{ "_id" : 5, "name" : "jalapeño", "menuCode" : "jal" }
{ "_id" : 6, "name" : "pizza", "menuCode" : "piz" }
{ "_id" : 7, "name" : "tacos", "menuCode" : "tac" }
{ "_id" : 8, "name" : "寿司sushi", "menuCode" : "寿" }
参见