In MongoDB, the $strcasecmp aggregation pipeline operator performs a case-insensitive comparison of two strings.

It returns either 1, 0, or -1, depending on whether or not the first string is greater than, equal to, or less than the second string.

Specifically, $strcasecmp returns:

• 1 if the first string is greater than the second string
• 0 if both strings are equal
• -1 if the first string is less than the second string

Example

Suppose we have a collection called data with the following documents:

{ "_id" : 1, "a" : "abc", "b" : "def" }
{ "_id" : 2, "a" : "abc", "b" : "abc" }
{ "_id" : 3, "a" : "def", "b" : "abc" }
{ "_id" : 4, "a" : "abc", "b" : "cba" }
{ "_id" : 5, "a" : "cba", "b" : "abc" }

Here’s what happens when we apply $strcasecmp to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 1, 2, 3, 4, 5 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $strcasecmp: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : "abc", "b" : "def", "result" : -1 }
{ "a" : "abc", "b" : "abc", "result" : 0 }
{ "a" : "def", "b" : "abc", "result" : 1 }
{ "a" : "abc", "b" : "cba", "result" : -1 }
{ "a" : "cba", "b" : "abc", "result" : 1 }

Case Sensitivity

As mentioned, $strcasecmp performs a case-insensitive comparison.

Suppose our collection contains the following document:

{ "_id" : 6, "a" : "ABC", "b" : "abc" }

The a field contains an uppercase string, and the b field contains the same string, but in lowercase.

Here’s what happens when we apply $strcasecmp to both fields:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 6 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $strcasecmp: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : "ABC", "b" : "abc", "result" : 0 }

The result is 0, which means that both strings are equal.

In other words, the comparison was case-insensitive.

Null Values

The $strcasecmp treats two null values as equal. Also, a string is considered greater than null .

Suppose we have the following documents in our collection:

{ "_id" : 7, "a" : "abc", "b" : null }
{ "_id" : 8, "a" : null, "b" : "abc" }
{ "_id" : 9, "a" : null, "b" : null }

Here’s what happens when we apply $strcasecmp to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 7, 8 ,9 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $strcasecmp: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : "abc", "b" : null, "result" : 1 }
{ "a" : null, "b" : "abc", "result" : -1 }
{ "a" : null, "b" : null, "result" : 0 }

Missing Fields

Missing fields have the same effect as null.

Let’s add the following documents to our collection:

{ "_id" : 10, "a" : "abc" }
{ "_id" : 11, "b" : "abc" }
{ "_id" : 12 }

Here’s what happens when we apply $strcasecmp to them:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 10, 11, 12 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $strcasecmp: [ "$a", "$b" ] }
          }
     }
   ]
)

Result:

{ "a" : "abc", "result" : 1 }
{ "b" : "abc", "result" : -1 }
{ "result" : 0 }

Other Data Types

Other data types can be compared, as long as they can resolve to a string.

Here are a bunch of documents that contain various data types:

{ "_id" : 13, "a" : 123, "b" : 456 }
{ "_id" : 14, "a" : 123, "b" : 123 }
{ "_id" : 15, "a" : 456, "b" : 123 }
{ "_id" : 16, "a" : NumberDecimal("123"), "b" : NumberDecimal("456") }
{ "_id" : 17, "a" : NumberDecimal("123"), "b" : NumberDecimal("123") }
{ "_id" : 18, "a" : NumberDecimal("456"), "b" : NumberDecimal("123") }
{ "_id" : 19, "a" : ISODate("1999-01-03T23:30:15.100Z"), "b" : "2000-01-03T23:30:15.100Z" }
{ "_id" : 20, "a" : ISODate("2000-01-03T23:30:15.100Z"), "b" : "2000-01-03T23:30:15.100Z" }
{ "_id" : 21, "a" : ISODate("2000-01-03T23:30:15.100Z"), "b" : "1999-01-03T23:30:15.100Z" }

And here’s what happens when we apply $strcasecmp to those documents:

db.data.aggregate(
   [
     { $match: { _id: { $in: [ 13, 14, 15, 16, 17, 18, 19, 20, 21 ] } } },
     {
       $project:
          {
            _id: 0,
            a: 1,
            b: 1,
            result: { $strcasecmp: [ "$a", "$b" ] }
          }
     }
   ]
).pretty()

Result:

{ "a" : 123, "b" : 456, "result" : -1 }
{ "a" : 123, "b" : 123, "result" : 0 }
{ "a" : 456, "b" : 123, "result" : 1 }
{ "a" : NumberDecimal("123"), "b" : NumberDecimal("456"), "result" : -1 }
{ "a" : NumberDecimal("123"), "b" : NumberDecimal("123"), "result" : 0 }
{ "a" : NumberDecimal("456"), "b" : NumberDecimal("123"), "result" : 1 }
{
	"a" : ISODate("1999-01-03T23:30:15.100Z"),
	"b" : "2000-01-03T23:30:15.100Z",
	"result" : -1
}
{
	"a" : ISODate("2000-01-03T23:30:15.100Z"),
	"b" : "2000-01-03T23:30:15.100Z",
	"result" : 0
}
{
	"a" : ISODate("2000-01-03T23:30:15.100Z"),
	"b" : "1999-01-03T23:30:15.100Z",
	"result" : 1
}