SQL Server: разница между PARTITION BY и GROUP BY

Question

Я использовал GROUP BY для всех типов агрегированных запросов на протяжении многих лет. Недавно я провел обратный инжиниринг кода, который использует PARTITION BY для выполнения агрегации. Читая всю документацию, которую я могу найти о PARTITION BY, это звучит очень похоже на GROUP BY, может быть, с добавлением немного дополнительной функциональности? Это две версии одного и того же общего функционала или они что-то совершенно другое?

Answer 1

-- BELOW IS A SAMPLE WHICH OUTLINES THE SIMPLE DIFFERENCES
-- READ IT AND THEN EXECUTE IT
-- THERE ARE THREE ROWS OF EACH COLOR INSERTED INTO THE TABLE
-- CREATE A database called testDB


-- use testDB
USE [TestDB]
GO


-- create Paints table
CREATE TABLE [dbo].[Paints](
    [Color] [varchar](50) NULL,
    [glossLevel] [varchar](50) NULL
) ON [PRIMARY]

GO


-- Populate Table
insert into paints (color, glossLevel)
select 'red', 'eggshell'
union
select 'red', 'glossy'
union
select 'red', 'flat'
union
select 'blue', 'eggshell'
union
select 'blue', 'glossy'
union
select 'blue', 'flat'
union
select 'orange', 'glossy'
union
select 'orange', 'flat'
union
select 'orange', 'eggshell'
union
select 'green', 'eggshell'
union
select 'green', 'glossy'
union
select 'green', 'flat'
union
select 'black', 'eggshell'
union
select 'black', 'glossy'
union
select 'black', 'flat'
union
select 'purple', 'eggshell'
union
select 'purple', 'glossy'
union
select 'purple', 'flat'
union
select 'salmon', 'eggshell'
union
select 'salmon', 'glossy'
union
select 'salmon', 'flat'


/*   COMPARE 'GROUP BY' color to 'OVER (PARTITION BY Color)'  */

-- GROUP BY Color 
-- row quantity defined by group by
-- aggregate (count(*)) defined by group by
select count(*) from paints
group by color

-- OVER (PARTITION BY... Color 
-- row quantity defined by main query
-- aggregate defined by OVER-PARTITION BY
select color
, glossLevel
, count(*) OVER (Partition by color)
from paints

/* COMPARE 'GROUP BY' color, glossLevel to 'OVER (PARTITION BY Color, GlossLevel)'  */

-- GROUP BY Color, GlossLevel
-- row quantity defined by GROUP BY
-- aggregate (count(*)) defined by GROUP BY
select count(*) from paints
group by color, glossLevel



-- Partition by Color, GlossLevel
-- row quantity defined by main query
-- aggregate (count(*)) defined by OVER-PARTITION BY
select color
, glossLevel
, count(*) OVER (Partition by color, glossLevel)
from paints