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Correct option is (c) $unwin

To EXPLAIN: $unwind operator allows for somewhat similar FUNCTIONALITY like JOIN, but with fields embedded within the DOCUMENT.

Right option is (c) $LIMIT

The BEST EXPLANATION: $limit takes a positive integer that SPECIFIES the maximum number of documents to pass along.

Right answer is (c) null

To EXPLAIN: If some, but not all, DOCUMENTS for the $min operation have either a null VALUE for the field or are missing the field, the $min operator only considers the non-null and the non-missing values for the field.

The CORRECT option is (d) None of the mentioned

For explanation I WOULD SAY: If the variable REFERENCES an object, to access a SPECIFIC field in the object, use the dot notation; i.e. “$$.”.

The CORRECT answer is (a) unique

The best EXPLANATION: ORDER of the ELEMENTS in the output array is unspecified.

The correct ANSWER is (B) { $min: }

Easiest explanation: $min returns the lowest value that results from applying an expression to each document in a group of documents that SHARE the same group by KEY.

Correct option is (B) $sort

To EXPLAIN: $sort takes a document that SPECIFIES the field(s) to sort by and the RESPECTIVE sort order.

The correct option is (B) $SUM

Explanation: $sum RETURNS a sum for each GROUP. IGNORES non-numeric values.

The CORRECT answer is (c) $avg ignores numeric values

Easy explanation: $avg returns the AVERAGE value of the numeric values that result from applying a specified expression to each DOCUMENT in a GROUP of documents that share the same group by key.

The CORRECT choice is (B) $ifNull

Explanation: $ifNull RETURNS the result of the SECOND expression if the first expression results in a NULL result.

The correct ANSWER is (C) NULL

Best explanation: Null value does not imply zero.

Right OPTION is (B) $dateToString

Best explanation: $dateToString converts a date OBJECT to a string ACCORDING to a user-specified format.

Right choice is (B) $hour

Easy explanation: The ARGUMENT in hour STATEMENT can be any EXPRESSION as long as it resolves to a DATE.

Correct answer is (B) $cond accepts either two expressions in an ordered LIST or three named parameters

The BEST I can explain: $cond is a ternary operator that EVALUATES one EXPRESSION, and depending on the result, returns the value of one of the other two expressions.

Right choice is (c) $millis RETURNS the milliseconds of a DATE as a NUMBER between 0 and 1000

Easy EXPLANATION: $MILLISECOND returns the milliseconds of a date as a number between 0 and 999.

Correct option is (a) jsMode

Easiest EXPLANATION: INTERNALLY, the JavaScript objects emitted during MAP FUNCTION remain as JavaScript objects.

Correct answer is (a) EMIT()

To EXPLAIN I would say: A single emit can only hold HALF of MongoDB’s maximum BSON document SIZE.

Right ANSWER is (a) BSON

For EXPLANATION I would SAY: These BSON objects are then converted back to JavaScript objects when calling the REDUCE function.

Correct OPTION is (b) scope

The BEST I can explain: It can ALSO ACCESS reduce functions.

Correct OPTION is (C) $out

For EXPLANATION I would say: During the post-processing, each shard will pull the results for its own CHUNKS from the other shards, run the final reduce/finalize, and write locally to the OUTPUT collection.

The correct CHOICE is (a) finalize

Explanation: Finalize FUCNTION is optional for MAPREDUCE OPERATION.

Right ANSWER is (d) None of the mentioned

The explanation: When using sharded collection as the input for a map-reduce OPERATION, mongos will automatically dispatch the map-reduce JOB to each shard in parallel.

The correct answer is (B) 2.0

To EXPLAIN: This MODEL REQUIRES only a small amount of memory, even for large data sets.

Correct answer is (b) The MAP-reduce operation is composed of many tasks

Easiest explanation: It includes reads from the input collection, executions of the map function, executions of the reduce function, writes to a temporary collection during PROCESSING, and writes to the OUTPUT collection.

Correct answer is (a) V8

The EXPLANATION: V8 JavaScript ENGINE became the default engine in 2.4 VERSION.

Correct ANSWER is (B) CURSOR

For explanation I would SAY: The aggregate command can return results as a cursor or store the results in a collection, which are not subject to the size LIMIT.

The correct choice is (B) $literal

The explanation is: USE for VALUES that the aggregation PIPELINE may interpret as an expression.

The correct choice is (C) $META

Easy explanation: $meta returns the metadata associated with a DOCUMENT in a PIPELINE operations.

The correct ANSWER is (d) None of the mentioned

Explanation: $LET defines variables for use within the SCOPE of a SUBEXPRESSION and returns the RESULT of the subexpression.

Right option is (d) NONE of the mentioned

Explanation: A $META EXPRESSION has the FOLLOWING syntax:{ $meta: }.

The CORRECT ANSWER is (a) $strcasecmpequals

To explain I would say: $strcasecmp returns 0 if two strings are EQUIVALENT.

Correct ANSWER is (B) $toLower CONVERTS a STRING to lowercase

Easy EXPLANATION: If the argument resolves to null, $toLower returns an empty string “”.

Right option is (c) $substr

For explanation: $substr accepts three expressions as arguments: the first ARGUMENT must resolve to a STRING, and the SECOND and third arguments must resolve to INTEGERS.