Introduction to MongoDB $concat Operator

The Mongodb $concat operator is used to concatenate strings. This operator can combine multiple strings into a single string.

Syntax

The syntax for the Mongodb $concat operator is as follows:

{ $concat: [ <expression1>, <expression2>, ... ] }

Here, <expression1>, <expression2>, and so on, are the string expressions that need to be concatenated. These expressions can be field names, string constants, or the result of other operators.

Use cases

The Mongodb $concat operator can be used in an aggregation pipeline to concatenate strings. It is typically used in the following scenarios:

• Creating a new string field in the query results.
• Combining multiple string fields into a single string.

Examples

The following example demonstrates how to use the Mongodb $concat operator to combine multiple strings into a single string.

Assume we have the following documents:

{ "_id": 1, "name": "John", "age": 20, "city": "New York" }
{ "_id": 2, "name": "Tom", "age": 25, "city": "London" }
{ "_id": 3, "name": "Jane", "age": 30, "city": "Paris" }

We can use the following aggregation pipeline to combine the name and city fields into a single string:

db.collection.aggregate([
  {
    $project: {
      fullName: { $concat: ["$name", ", ", "$city"] }
    }
  }
])

After running the above aggregation pipeline, we get the following results:

{ "_id": 1, "fullName": "John, New York" }
{ "_id": 2, "fullName": "Tom, London" }
{ "_id": 3, "fullName": "Jane, Paris" }

Conclusion

The Mongodb $concat operator is a very useful operator that can be used to combine multiple strings into a single string, making it easy to process data. In an aggregation pipeline, using the $concat operator can create new string fields or combine multiple string fields into a single string.