MS SQL a RANK funkcie

Október 2022

V SQL rank funkcie umožňujú učiť poradie v dopyte podľa nejakého kritéria. Táto funkcia je veľmi užitočná pri istom type dopytov. V tomto blogu píšem aj o iných maličkostiach, uľahčujúcich život. No nejde o žiadne novinky, plno týchto funkcionalít je dostupný od SQL Serveru 2008 R2.

Príkladový problém

Keď som ešte pracoval s PHP a MySQL na sociálnej sieti pre lukostrelcov, tak som dostal úlohu, zobraziť v detaile luku (produktu) najlepšie výsledky, ktoré tým lukom boli dosiahnuté v jednotlivých kategóriách súťaží, kde boli použité.

Šlo to spraviť selfjoinom a vnoreným dopytom, no to MySQL zabilo, doslova, pri tabuľke s dvoma tisíckami záznamov vyletel procesor na 100% a po troch minútach MySQL zamrzla a bolo potrebné ju celú reštartovať. Riešením bolo nakoniec použiť N+1 dopytov – najskôr sa vytiahol zoznam kategórií a potom sa pre každú vytiahli najlepšie výsledky so súťažou a súťažiacim.

Presne na takýto scenár ide požiť funkcia RANK (DENSE_RANK) v MS SQL.

Nasledujúci kód vytvorí testovaciu databázu, nad ktorou ukážem riešenie podobného problému ako vyššie a tiež použitie window funkcií.

CREATE TABLE [dbo].[Competition](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[Name] [nvarchar](500) NOT NULL,
 CONSTRAINT [PK_Competition] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[Competition_CompetitionCathegory](
	[CompetitionId] [int] NOT NULL,
	[CompetitionCathegoryId] [int] NOT NULL,
 CONSTRAINT [PK_Competition_CompetitionCathegory] PRIMARY KEY CLUSTERED 
(
	[CompetitionId] ASC,
	[CompetitionCathegoryId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[CompetitionCategory](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[Name] [nvarchar](50) NOT NULL,
 CONSTRAINT [PK_CompetitionCategory] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[CompetitionResult](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[UserId] [int] NOT NULL,
	[CompetitionId] [int] NOT NULL,
	[CathegoryId] [int] NOT NULL,
	[Points] [int] NULL,
	[Notes] [nvarchar](500) NULL,
 CONSTRAINT [PK_CompetitionResult] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[User](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[Name] [nvarchar](50) NOT NULL,
	[Gender] [bit] NULL,
 CONSTRAINT [PK_User] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[Competition_CompetitionCathegory]  WITH CHECK ADD  CONSTRAINT [FK_Competition_CompetitionCathegory_Competition] FOREIGN KEY([CompetitionId])
REFERENCES [dbo].[Competition] ([Id])
GO
ALTER TABLE [dbo].[Competition_CompetitionCathegory] CHECK CONSTRAINT [FK_Competition_CompetitionCathegory_Competition]
GO
ALTER TABLE [dbo].[Competition_CompetitionCathegory]  WITH CHECK ADD  CONSTRAINT [FK_Competition_CompetitionCathegory_CompetitionCategory] FOREIGN KEY([CompetitionCathegoryId])
REFERENCES [dbo].[CompetitionCategory] ([Id])
GO
ALTER TABLE [dbo].[Competition_CompetitionCathegory] CHECK CONSTRAINT [FK_Competition_CompetitionCathegory_CompetitionCategory]
GO
ALTER TABLE [dbo].[CompetitionResult]  WITH CHECK ADD  CONSTRAINT [FK_CompetitionResult_Competition] FOREIGN KEY([CompetitionId])
REFERENCES [dbo].[Competition] ([Id])
GO
ALTER TABLE [dbo].[CompetitionResult] CHECK CONSTRAINT [FK_CompetitionResult_Competition]
GO
ALTER TABLE [dbo].[CompetitionResult]  WITH CHECK ADD  CONSTRAINT [FK_CompetitionResult_CompetitionCategory] FOREIGN KEY([CathegoryId])
REFERENCES [dbo].[CompetitionCategory] ([Id])
GO
ALTER TABLE [dbo].[CompetitionResult] CHECK CONSTRAINT [FK_CompetitionResult_CompetitionCategory]
GO
ALTER TABLE [dbo].[CompetitionResult]  WITH CHECK ADD  CONSTRAINT [FK_CompetitionResult_User] FOREIGN KEY([UserId])
REFERENCES [dbo].[User] ([Id])
GO
ALTER TABLE [dbo].[CompetitionResult] CHECK CONSTRAINT [FK_CompetitionResult_User]
GO

SET IDENTITY_INSERT [dbo].[CompetitionCategory] ON
GO
INSERT [dbo].[CompetitionCategory] ([Id], [Name]) VALUES (1, N'Bowhunter Freestyle Limited'), (2, N'Barebow'), (3, N'Freestyle Limited'),
(4, N'Traditional'), (5, N'NFAA'), (6, N'Indoor')
GO
SET IDENTITY_INSERT [dbo].[CompetitionCategory] OFF
GO

SET IDENTITY_INSERT [dbo].[Competition] ON 
GO
INSERT [dbo].[Competition] ([Id], [Name]) VALUES (1, N'Minnehaha Archers Summer Field League - Week 5'),
(2, N'Face2Face'),
(3, N'Minnehaha Archers Summer Field League - Week 7')
GO
SET IDENTITY_INSERT [dbo].[Competition] OFF
GO
INSERT [dbo].[Competition_CompetitionCathegory] ([CompetitionId], [CompetitionCathegoryId]) VALUES (1, 1),(1, 2),(1, 3),(2, 2),(2, 3),(2, 4),(3, 5),(3, 6)
GO
SET IDENTITY_INSERT [dbo].[User] ON 
GO
INSERT [dbo].[User] ([Id], [Name], [Gender]) VALUES (1, N'Lena', 0),
(2, N'Jano', 1),(3, N'Dano', 1),(4, N'Marianna', 0),
(5, N'Greta', 0),(6, N'Ada', 0),
(7, N'Mirka', 0),(8, N'Peter', 1),
(9, N'Martin', 1),(10, N'Martin G.', 1),
(11, N'Maria', 0),(12, N'Silvester', 1)
GO
SET IDENTITY_INSERT [dbo].[User] OFF
GO
SET IDENTITY_INSERT [dbo].[CompetitionResult] ON
INSERT [dbo].[CompetitionResult] ([Id], [UserId], [CompetitionId], [CathegoryId], [Points], [Notes]) VALUES (1, 1, 1, 1, 256, NULL),
(2, 2, 1, 1, 120, NULL),(3, 3, 1, 1, 1500, NULL),(4, 4, 1, 2, 41, NULL),
(5, 2, 1, 2, 24, NULL),(6, 5, 1, 2, 24, NULL),(7, 1, 1, 2, 0, NULL),
(8, 6, 1, 2, 15, NULL),(9, 2, 1, 3, 120, NULL),(10, 3, 1, 3, 110, NULL),
(11, 6, 1, 4, 130, NULL),(12, 7, 2, 4, 80, NULL),(13, 8, 2, 4, 114, NULL),
(14, 7, 2, 2, 85, NULL),(15, 9, 2, 2, 91, NULL),(16, 10, 2, 2, 42, NULL),
(17, 12, 2, 2, 91, NULL),(18, 6, 2, 3, 174, NULL),(19, 7, 2, 3, 200, NULL),
(20, 8, 2, 3, 195, NULL),(21, 9, 2, 3, 112, NULL),(22, 10, 2, 3, 45, NULL),
(23, 11, 2, 3, 138, NULL),(24, 12, 2, 3, 0, NULL),(25, 7, 3, 5, 47, NULL),
(26, 8, 3, 5, 39, NULL),
(27, 1, 3, 6, 266, NULL),(28, 2, 3, 6, 200, NULL),(29, 3, 3, 6, 1101, NULL)
GO
SET IDENTITY_INSERT [dbo].[CompetitionResult] OFF
GO

Pre vytiahnutntie súťažiacich, ktorý dosiahli najviac bodov v danej kategórii ide použiť nasledujúce SQL.

Je v ňom využité CTE a funkcia DENSE_RANK, ktorá očísluje riadky podľa počtu bodov pre konkrétnu kategóriu. Následne je vonkajšou podmienkou zabezpečené aby sa vybrali len najlepšie výsledky.

Toto SQL je jednoduché upraviť tak, aby vracalo N najlepších výsledkov pomocou úpravy WHERE klauzuly.

WITH RankedPoints ([UserName], [Competition], [Cathegory], [Points], [Rank])
AS
(
 SELECT [u].[Name] AS [UserName],
        [c].[Name] AS [Competition],
        [cc].[Name] AS [Cathegory],
        [cr].[Points] AS [Points],
	    DENSE_RANK() OVER (PARTITION BY [cr].[CathegoryId] ORDER BY [cr].[Points] DESC) AS [Rank]
  FROM [dbo].[CompetitionResult] [cr]
  LEFT JOIN [dbo].[User] [u] ON [cr].[UserId] = [u].[Id]
  LEFT JOIN [dbo].[Competition] [c] ON [cr].[CompetitionId] = [c].[Id]
  LEFT JOIN [dbo].[CompetitionCategory] [cc] ON [cr].[CathegoryId] = [cc].[Id]
)
SELECT [UserName], [Competition], [Cathegory], [Points] FROM RankedPoints
WHERE [Rank] = 1

Iné triky

Window funkcie umožňujú „naporcovať“ skúmané dáta podobne ako pri grupovaní, ale týmto spôsobom je možné vytiahnuť aj iné údaje v riadkoch a „porcoať“ ich podľa rôznych kritérií.

Nasledujúci selekt vráti medián bodov a počet súťažiacich pre kategórie. Window funkcie sa zvyčajne používajú s DISTINCT kvôli opakujúcim sa riadkom vo výsledkoch.

SELECT DISTINCT [cc].[Name] AS [Cathegory],
		PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY  [Points]) OVER (PARTITION BY [cr].[CathegoryId]) AS [PointsMedian],
		COUNT([cr].[UserId]) OVER (PARTITION BY [cr].[CathegoryId]) AS [Count]
  FROM [dbo].[CompetitionResult] [cr]
  LEFT JOIN [dbo].[User] [u] ON [cr].[UserId] = [u].[Id]
  LEFT JOIN [dbo].[CompetitionCategory] [cc] ON [cr].[CathegoryId] = [cc].[Id]

Do agregačných funkcií idú vpísať podmienky, to sa dá využiť pri rôznom spočítavaní. Napríklad nasledujúci dopyt vráti počet súťažiacich mužov a žien v kategóriách:

SELECT [cc].[Name] AS [Cathegory],
	   COUNT(*) AS [Total],
	   SUM(IIF([u].[Gender] = 1, 1, 0)) AS [Men],
	   SUM(IIF([u].[Gender] = 0, 1, 0)) AS [Women]
  FROM [dbo].[CompetitionResult] [cr]
  LEFT JOIN [dbo].[User] [u] ON [cr].[UserId] = [u].[Id]
  LEFT JOIN [dbo].[CompetitionCategory] [cc] ON [cr].[CathegoryId] = [cc].[Id]
  GROUP BY [cr].[CathegoryId], [cc].[Name]

Záver

Tento blog nemal za cieľ byť úplným a a vyčerpávajúcim, skôr šlo ukázať niektoré málo známe možnosti.