COUNT (U-SQL)

Artigo
03/10/2017

Summary

The COUNT aggregator returns the number of items in a group. If COUNT(*) is specified the count will include null values, if an expression or column reference is provided, then null values will not be counted.

The identity value is 0.

Syntax

COUNT_Expression := 
    'COUNT' '(' ['DISTINCT'] (expression | '*') ')'.

Remarks

DISTINCT
Optionally allows to de-duplicate the values returned by the expression inside the group before aggregation.
expression | *
The C# expression (including column references) for which the nonnull values in a group get counted or * to indicate to count the rows and include null values in the count.

Return Type

The return type is long?.

Usage in Windowing Expression

This aggregator can be used in a windowing expression without any additional restrictions.

Examples

The examples can be executed in Visual Studio with the Azure Data Lake Tools plug-in.
The scripts can be executed locally. An Azure subscription and Azure Data Lake Analytics account is not needed when executed locally.
The examples below are based on the dataset defined below. Ensure you create and populate the table in a separate window before executing the examples.

DROP TABLE IF EXISTS master.dbo.Employees;

CREATE TABLE master.dbo.Employees
(
    EmpID int,
    EmpName string,
    DeptName string,
    DeptID int,
    Salary int,
    INDEX cl_idx CLUSTERED(EmpID ASC) PARTITIONED BY HASH(EmpID)
);

INSERT INTO master.dbo.Employees
(EmpID, EmpName, DeptName, DeptID, Salary)
VALUES
(1, "Noah",   "Engineering", 100, 10000),
(2, "Sophia", "Engineering", 100, 20000),
(3, "Liam",   "Engineering", 100, 30000),
(4, "Emma",   "HR",          200, 10000),
(5, "Jacob",  "HR",          200, 10000),
(6, "Olivia", "HR",          200, 10000),
(7, "Mason",  "Executive",   300, 50000),
(8, "Ava",    "Marketing",   400, 15000),
(9, "Ethan",  "Marketing",   400, 10000);

INSERT INTO master.dbo.Employees
(EmpID, EmpName, DeptID, Salary)
VALUES
(10, "David", 800, 100);

A. Count of all values
The following query calculates the: 1) total record count, 2) total department count, and 3) distinct number of departments.

@result =
    SELECT COUNT( * ) AS TotalRecordCount,
           COUNT(DeptName) AS DeptNameCount,
           COUNT(DISTINCT DeptName) AS DistinctDeptNameCount
    FROM master.dbo.Employees;

OUTPUT @result
TO "/Output/ReferenceGuide/count/exampleA.csv"
USING Outputters.Csv();

B. Count values per group
The following query calculates the count of employees within each department with the GROUP BY clause.

@result =
    SELECT DeptName,
           COUNT( * ) AS EmpNameByDeptCount
    FROM master.dbo.Employees
    GROUP BY DeptName;

OUTPUT @result
TO "/Output/ReferenceGuide/count/exampleB.csv"
USING Outputters.Csv();

C. Count values with OVER()
The OVER clause in the following query is empty which defines the "window" to include all rows. The query calculates the employee count over the window - all employees.

@result =
    SELECT EmpName,
           COUNT( * ) OVER() AS EmpNameCount
    FROM master.dbo.Employees;

OUTPUT @result
TO "/Output/ReferenceGuide/count/exampleC.csv"
USING Outputters.Csv();

D. Count values over a defined window using OVER()
The OVER clause in the following query is DeptName. The query returns all records and the number of employees over the window - DeptName.

@result =
    SELECT *,
           COUNT( * ) OVER(PARTITION BY DeptName) AS EmpNameByDeptCount
    FROM master.dbo.Employees;

OUTPUT @result
TO "/Output/ReferenceGuide/count/exampleD.csv"
USING Outputters.Csv();

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