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- $substr (aggregation)
$substr (aggregation)¶
- $substr¶
Returns a substring of a string, starting at a specified index position and including the specified number of characters. The index is zero-based.
$substr has the following syntax:
{ $substr: [ <string>, <start>, <length> ] }
The arguments can be any valid expression as long as long as the first argument resolves to a string, and the second and third arguments resolve to integers. For more information on expressions, see Expressions.
Behavior¶
If <start> is a negative number, $substr returns an empty string "".
If <length> is a negative number, $substr returns a substring that starts at the specified index and includes the rest of the string.
$substr is not encoding aware and if used improperly may produce a result string containing an invalid UTF-8 character sequence.
$substr only has a well-defined behavior for strings of ASCII characters.
Example¶
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 $substr operator separate the quarter value into a yearSubstring and a quarterSubstring:
db.inventory.aggregate(
[
{
$project:
{
item: 1,
yearSubstring: { $substr: [ "$quarter", 0, 2 ] },
quarterSubtring: { $substr: [ "$quarter", 2, -1 ] }
}
}
]
)
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" }