In multi-threaded POSIX mode, there is a method called a pipeline, whose data element stream is executed sequentially by a set of threads in the specified order. $ first – Gets only the first document from the grouped ones, usually for sorting.$ addToSet – Inserts a value into an array of the resulting document, but does not create duplicates.$ push – inserts the field value into the result field.$ max – Get the maximum value from the numeric field.$ min – Returns the minimum value from the numeric field.$ avg – Calculates the average between numeric fields.
$ sum – Returns the sum of all numeric fields.To understand the MongoDB group by multiple fields first, let’s have a look at a list of all operators that can be used in $ group:
It has its own operator, we can get the field of the current document by $ symbol + field name. The $ group operator is an aggregator that returns a new document. MongoDB offers a very powerful aggregation operation that can be divided into three categories: The input of the aggregation operation in MongoDB is the collection document. Aggregation operations can perform complex collections operations, especially for math statistics and data mining.
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After the collection is grouped by condition, a series of operations such as sum, average, and others are performed. The MongoDB aggregation operation is used for batch operations on the data. That’s why the aggregate function must use the GROUP BY clause to connect to the list in the SELECT list.įor example, instead of using the GROUP BY clause, AVG in a SELECT list can only correspond to SUM, but it cannot correspond to a specific column. If you do not use the GROUP BY clause, some aggregate functions in the SELECT list can only be used with other aggregate functions. When an aggregate function is used in a query without the GROUP BY clause, the aggregate function aggregates the entire result set (all rows that match the WHERE clause). However, the use of aggregate function is not limited to grouped queries. Aggregate functions perform better when used with the GROUP BY clause. MongoDB Aggregation FunctionĪggregation functions are used in GROUP BY clauses to aggregate grouped data. To understand better first we will learn about the aggregation function. In this article I will introduce you to the Mongodb group by multiple fields.
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