$concat (aggregation)
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Definition
$concat
Concatenates strings and returns the concatenated string.
$concat
has this syntax:{ $concat: [ <expression1>, <expression2>, ... ] } The arguments can be any valid expression as long as they resolve to strings. For more information on expressions, see Expression Operators.
If the argument resolves to a value of
null
or refers to a field that is missing,$concat
returnsnull
.
Example
Create an inventory
collection with these documents:
db.inventory.insertMany( [ { _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 } ] )
Use the $concat
operator to concatenate the item
field and the
description
field with a " - " delimiter:
db.inventory.aggregate( [ { $project: { itemDescription: { $concat: [ "$item", " - ", "$description" ] } } } ] )
Output:
{ _id : 1, itemDescription : "ABC1 - product 1" } { _id : 2, itemDescription : "ABC2 - product 2" } { _id : 3, itemDescription : null }