mongodb aggregate functions

, which is an open-source document database system with the development language C ++. It provides an efficient document-oriented storage structure and supports processing stored documents through MapReduce programs. It is highly scalable and supports automatic partitioning. Mapreduce can be used for data aggregation. Its data is stored in BSON (Binary JSON) format. Its storage structure supports dynamic schema and supports dynamic query. Unlike rdbms SQL queries, the Mongo query language is rep

Mongodb Aggregate functions 1. the count function is very understandable. Like other databases, it is used to perform the count operation selectcount (*) fromtab1db. the tab1.count () and above examples are quite understandable, but if there are operations such as skip and limit in the operation, count will ignore these operations and a true parameter must be add

This article introduced the MongoDB in the MapReduce implementation of data aggregation method, we mentioned MongoDB in the data aggregation operation of a way--mapreduce, but in most of the day-to-day use of the process, We do not need to use MapReduce to do the operation. In this article, we simply talk about the implementation of the data aggregation operation with the aggregation function. In addition

number.Select rand () -- generate 0 ~ Random Number of 1 (the system time is used as the seed by default)Select rand (50) -- generate a fixed value (seed is set to a fixed 50)---------------- System functions ------------------------- Convert (target data type, which must be converted): Data Type ConversionSelect 1 + 2 -- addition operationSelect '1' + '2' -- character connectionSelect '1' + 2 -- display 3Select '1' + convert (varchar, 2) -- display

Sometimes we need to summarize the data in the table, instead of the data itself. To facilitate these types of retrieval, SQL provides five Aggregate functions, SQL Aggregate functions are quite consistent in all major SQL implementations. As follows: the AVG () function of 1.1AVG () calculates the sum of its column va

sql--aggregate function (Aggregate functions): Avg,count,first,last,max,min,sumavg () functionDefinition and UsageThe AVG function returns the average of a numeric column. NULL values are not included in the calculation.SQL AVG () syntaxSELECT AVG (column_name) from table_nameSQL AVG () instanceWe have the following "Orders" table: o_id Order

from sysdate) | 'month' | extract (day from sysdate) | 'day' as converted date from dual -- to_char (): date conversion format, default "DD-MON-YY" select to_char (sysdate) from dual; select to_char (sysdate, 'yyyy "year" mm "month" DD "day" hh ":" mi ":" ss') from dual; -- 12-hour format select to_char (sysdate, 'yyyy "-" mm "-" DD "-" hh24 ":" mi ":" SS ") from dual; -- 24-hour format -- decode (): it is equivalent to multiple if-else (taking the EMP table as an example) Select empno as numbe

SQL has many built-in functions that can be used for counting and computation.Syntax of the functionThe syntax for the built-in SQL function is:SELECT function (column) from tableTypes of functionsIn SQL, there are several kinds of basic function types and types. The basic types of functions are: Aggregate function Scalar function

Oracle date functions/character functions/numeric functions/conversion functions/Aggregate functions Oracle date functions: [SQL] MONTHS_BETWEEN: returns the difference of the month bet

background by adding the background:true option when creating the index*/ /********************************************************************************************************** ***//********************************************************************************************************** ***/ /*** MongoDB Aggregation*/ /*** MongoDB Aggregation (aggregate) i

information Functions The commonly used information functions are: (1) View database connection ID and current database SELECT connection_id ();SELECT DATABASE (); (2) Check the ID number of the last input record ALTER TABLE Test ADD ID SMALLINT UNSIGNED keyauto_increment first;INSERT Test (First_name,last_name) VALUES (' 11 ', ' 22 ');SELECT last_insert_id ();SELECT * from Test; (3) View current user and v

In MongoDB, there are three ways to do this, but most of the three are silly, this article is to sum up the difference between the following three ways ~ 1. Polymeric Framework Aggregate Pipeline 2. MapReduce 3. Aggregation command Group,distinct,count Aggregation framework aggregate pipeline (aggregation pipeline) The aggreg