Filtrování pomocí WHERE

Improving Query Performance in SQL Server

Dean Smith

Founder, Atamai Analytics

Jak funguje WHERE

SELECT *
FROM PlayerStats
WHERE Position = 'SG'

Tabulka PlayerStats

Jak funguje WHERE

SELECT *
FROM PlayerStats
WHERE Position = 'SG'

Filtrovaná tabulka PlayerStats

Pořadí zpracování WHERE

SELECT PlayerName, 
      Team, 
      (DRebound+ORebound) AS TotalRebounds
FROM PlayerStats
WHERE TotalRebounds >= 1000
ORDER BY TotalRebounds DESC;

WHERE se zpracovává před SELECT

-- ERROR
Invalid column name 'TotalRebounds'.

Použití poddotazu

SELECT PlayerName, 
       Team, 
       TotalRebounds
FROM
     -- Start of sub-query tr
    (SELECT PlayerName, Team, 
             (DRebound+ORebound) AS TotalRebounds
     FROM PlayerStats) tr
WHERE TotalRebounds >= 1000 -- created in the sub-query
ORDER BY TotalRebounds DESC;

Použití poddotazu

SELECT PlayerName, 
       Team, 
       TotalRebounds
FROM
     -- Start of sub-query tr
    (SELECT PlayerName, Team, 
             (DRebound+ORebound) AS TotalRebounds
     FROM PlayerStats) tr
WHERE TotalRebounds >= 1000 -- created in the sub-query
ORDER BY TotalRebounds DESC;

PlayerName	Team	TotalRebounds
Andre Drummond	DET	1247
DeAndre Jordan	LAC	1171
Karl-Anthony Towns	MIN	1012
Dwight Howard	CHO	1012

Výpočty na sloupcích

SELECT PlayerName, 
       Team, 
       (DRebound+ORebound) AS TotalRebounds
FROM PlayerStats
WHERE (DRebound+ORebound) >= 1000
ORDER BY TotalRebounds DESC;

Výpočty na sloupcích v podmínce filtru WHERE mohou prodloužit dobu dotazu

PlayerName	Team	TotalRebounds
Andre Drummond	DET	1247
DeAndre Jordan	LAC	1171
Karl-Anthony Towns	MIN	1012
Dwight Howard	CHO	1012

Funkce na sloupcích

SELECT PlayerName, College, DraftYear 
FROM Players
WHERE UPPER(LEFT(College,7)) = 'GEORGIA'; 
-- unnecessary use of functions 
-- on a filtering column

Použití funkcí na sloupcích v podmínce filtru WHERE může prodloužit dobu dotazu

PlayerName	College	DraftYear
Damien Wilkins	Georgia
Derrick Favors	Georgia Tech	2010
Iman Shumpert	Georgia Tech	2011
R.J. Hunter	Georgia State	2015
...	...	...

Zjednodušení WHERE

SELECT PlayerName, College, DraftYear 
FROM Players 
        -- No calculation or function
WHERE College like 'Georgia%';

PlayerName	College	DraftYear
Damien Wilkins	Georgia
Derrick Favors	Georgia Tech	2010
Iman Shumpert	Georgia Tech	2011
R.J. Hunter	Georgia State	2015
...	...	...

Shrnutí

WHERE se zpracovává před SELECT
Výpočty na sloupcích v podmínce filtru WHERE mohou prodloužit dobu dotazu
Použití funkcí na sloupcích v podmínce filtru WHERE může prodloužit dobu dotazu

Vyzkoušejte si to!

Improving Query Performance in SQL Server