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- $concat (aggregation)
$concat (aggregation)¶
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Definition¶
- $concat¶
New in version 2.4.
Concatenates strings and returns the concatenated string.
$concat has the following syntax:
{ $concat: [ <expression1>, <expression2>, ... ] }
The arguments can be any valid expression as long as they resolve to strings. For more information on expressions, see Expressions.
If the argument resolves to a value of null or refers to a field that is missing, $concat returns null.
Examples¶
Consider a 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 $concat operator to concatenate the item field and the description field with a `` - `` delimiter.
db.inventory.aggregate(
[
{ $project: { itemDescription: { $concat: [ "$item", " - ", "$description" ] } } }
]
)
The operation returns the following results:
{ "_id" : 1, "itemDescription" : "ABC1 - product 1" }
{ "_id" : 2, "itemDescription" : "ABC2 - product 2" }
{ "_id" : 3, "itemDescription" : null }
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