$and (aggregation)

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Definition

Behavior
Error Handling
Example

Definition

$and

Evaluates one or more expressions and returns true if all of the expressions are true or if run with no argument expressions. Otherwise, $and returns false.

$and syntax:

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

For more information on expressions, see Expression Operators.

Behavior

In addition to the false boolean value, $and evaluates as false the following: null, 0, and undefined values. The $and evaluates all other values as true, including non-zero numeric values and arrays.

Example	Result
`{ $and: [ 1, "green" ] }`	`true`
`{ $and: [ ] }`	`true`
`{ $and: [ [ null ], [ false ], [ 0 ] ] }`	`true`
`{ $and: [ null, true ] }`	`false`
`{ $and: [ 0, true ] }`	`false`

Error Handling

To allow the query engine to optimize queries, $and handles errors as follows:

If any expression supplied to $and would cause an error when evaluated alone, the $and containing the expression may cause an error but an error is not guaranteed.
An expression supplied after the first expression supplied to $and may cause an error even if the first expression evaluates to false.

For example, the following query always produces an error if $x is 0:

db.example.find( {
   $expr: { $eq: [ { $divide: [ 1, "$x" ] }, 3 ] }
} )

The following query, which contains multiple expressions supplied to $and, may produce an error if there is any document where $x is 0:

db.example.find( {
   $and: [
      { x: { $ne: 0 } },
      { $expr: { $eq: [ { $divide: [ 1, "$x" ] }, 3 ] } }
   ]
} )

Example

Create an example inventory collection with these documents:

db.inventory.insertMany([
   { "_id" : 1, "item" : "abc1", description: "product 1", qty: 300 },
   { "_id" : 2, "item" : "abc2", description: "product 2", qty: 200 },
   { "_id" : 3, "item" : "xyz1", description: "product 3", qty: 250 },
   { "_id" : 4, "item" : "VWZ1", description: "product 4", qty: 300 },
   { "_id" : 5, "item" : "VWZ2", description: "product 5", qty: 180 }
])

This operation uses the $and operator to determine if qty is greater than 100 and less than 250:

db.inventory.aggregate(
   [
     {
       $project:
          {
            item: 1,
            qty: 1,
            result: { $and: [ { $gt: [ "$qty", 100 ] }, { $lt: [ "$qty", 250 ] } ] }
          }
     }
   ]
)

The operation returns these results:

{ "_id" : 1, "item" : "abc1", "qty" : 300, "result" : false }
{ "_id" : 2, "item" : "abc2", "qty" : 200, "result" : true }
{ "_id" : 3, "item" : "xyz1", "qty" : 250, "result" : false }
{ "_id" : 4, "item" : "VWZ1", "qty" : 300, "result" : false }
{ "_id" : 5, "item" : "VWZ2", "qty" : 180, "result" : true }

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