MongoDB $concat Operator

What is the $concat operator in MongoDB?

MongoDB provides a variety of string expression operators that are used in the aggregation pipeline stages. The $concat operator is one of those operators. The $concat operator is used to concatenate two or more strings into a single string.

Syntax of the $concat operator:

The argument passed in this operator can be any valid expression until it resolves to the string.

If the argument's value refers to a missing field or null, the $concat operator returns null.

Examples:

Suppose we have a collection of the students with the following documents.

>db.students.find().pretty()
{
  {
	"_id" : 1,
	"name" : "Steve",
	"surname" : "Smith",
	"department" : "B-tech",
	"fees" : 80000
  }
  {
	"_id" : 2,
	"name" : "Sandy",
	"surname" : "Beach",
	"department" : "BCA",
	"fees" : 55000
  }
  {
	"_id" : 3,
	"name" : "John",
	"surname" : "Cena",
	"department" : "MCA",
	"fees" : 85000
  }
  {
	"_id" : 4,
	"name" : "Wick",
	"surname" : "John",
	"department" : "B.com",
	"fees" : 60000
  }
  {
	"_id" : 5,
	"name" : "David",
	"surname" : "Silva",
	"department" : "null",
	"fees" : 80000
  }
}

Example 1: Using $concat operator

In this example, we're going to concatenate the name field with surname field using the $concat operator.

db.students.aggregate(
  [
    { $project : { 
                          _id : 1,
                          name : { $concat: [ "$firstName", "$surname" ] },
                          department : 1,
                          fees : 1
                        }
    }
  ]
)

Output:

{
	"_id" : 1,
	"name" : "SteveSmith",
	"department" : "B-tech",
	"fees" : 80000
}
{
	"_id" : 2,
	"name" : "SandyBeach",
	"department" : "BCA",
	"fees" : 55000
}
{
	"_id" : 3,
	"name" : "JohnCena",
	"department" : "MCA",
	"fees" : 85000
}
{
	"_id" : 4,
	"name" : "WickJohn",
	"department" : "B.com",
	"fees" : 60000
}
{
	"_id" : 5,
	"name" : "DavidSilva",
	"department" : "null",
	"fees" : 80000
}

Suppose we add the following documents to our collection:

{
	"_id" : 6,
	"name" : "null",
	"surname" : "Thomas",
	"department" : "M.com",
	"fees" : 60000
}
{
	"_id" : 7,
	"name" : "David",
	"surname" : "null",
	"department" : "BCA",
	"fees" : 60000
}

Example 2: Null Values

In this example, we're going to concatenate the name field with surname field using the $concat operator.

db.students.aggregate(
  [
   { $match : { _id : { $nin: [ 1, 2, 3, 4, 5 ] } } },
      { $project : { 
                          _id : 1,
                          name : { $concat: [ "$firstName", "$surname" ] },
                          department : 1,
                          fees : 1
                        }
    }
  ]
)

Output:

{
	"_id" : 6,
	"name" : null,
	"department" : "M.com",
	"fees" : 60000
}
{
	"_id" : 7,
	"name" : null,
	"department" : "BCA",
	"fees" : 60000
}

Example 3: More than two string

In this example, we're going to concatenate the name field with surname using the $concat operator.

db.students.aggregate(
  [
    { $match : { _id : { $nin: [ 4, 5, 6, 7 ] } } },
       { $project : { 
                          _id : 1,
                          name : { $concat: [ "$firstName", " ", "$surname" ] },
                          department : 1,
                          fees : 1
                        }
    }
  ]
)

Output:

{
	"_id" : 1,
	"name" : "Steve Smith",
	"department" : "B-tech",
	"fees" : 80000
}
{
	"_id" : 2,
	"name" : "Sandy Beach",
	"department" : "BCA",
	"fees" : 55000
}
{
	"_id" : 3,
	"name" : "John Cena",
	"department" : "MCA",
	"fees" : 85000
}

Another example:

db.students.aggregate(
  [
    { $match : { _id : { $nin: [ 4, 5, 6, 7 ] } } },
       { $project : { 
                             _id : 1,
                             Student : { $concat: [ "$firstName", " ", "$surname", ", ", "$department"] },
                             fees : 1
                           }
    }
  ]
)

Output:

{
	"_id" : 1,
	"Student" : "Steve Smith, B-tech",
	"fees" : 80000
}
{
	"_id" : 2,
	"Student" : "Sandy Beach, BCA",
	"fees" : 55000
}
{
	"_id" : 3,
	"Student" : "John Cena, MCA",
	"fees" : 85000
}

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