Query order

Improving Query Performance in SQL Server

Dean Smith

Founder, Atamai Analytics

Big earthquakes query

SELECT Country, Place, Magnitude
FROM Earthquakes
WHERE Magnitude >= 9
ORDER BY Magnitude DESC;

Country	Place	Magnitude
CL	Bio-Bio; Chile	9.5
US	Southern Alaska	9.2
ID	off the west coast of northern Sumatra	9.1
JP	near the east coast of Honshu; Japan	9.1
...	...	...

Syntax order

                                 -- Syntax Order
SELECT Country, Place, Magnitude -- 1. SELECT
FROM Earthquakes                                            
WHERE Magnitude >= 9            
ORDER BY Magnitude DESC;

Syntax order

                                 -- Syntax Order
SELECT Country, Place, Magnitude -- 1. SELECT
FROM Earthquakes                 -- 2. FROM                            
WHERE Magnitude >= 9            
ORDER BY Magnitude DESC;

Syntax order

                                 -- Syntax Order
SELECT Country, Place, Magnitude -- 1. SELECT
FROM Earthquakes                 -- 2. FROM                            
WHERE Magnitude >= 9             -- 3. WHERE
ORDER BY Magnitude DESC;

Syntax order

                                 -- Syntax Order
SELECT Country, Place, Magnitude -- 1. SELECT
FROM Earthquakes                 -- 2. FROM                            
WHERE Magnitude >= 9             -- 3. WHERE
ORDER BY Magnitude DESC;         -- 4. ORDER BY

Processing order

                                 -- Syntax Order | Processing Order
SELECT Country, Place, Magnitude -- 1. SELECT        
FROM Earthquakes                 -- 2. FROM        1. FROM                       
WHERE Magnitude >= 9             -- 3. WHERE
ORDER BY Magnitude DESC;         -- 4. ORDER BY

Processing order

                                 -- Syntax Order | Processing Order
SELECT Country, Place, Magnitude -- 1. SELECT        
FROM Earthquakes                 -- 2. FROM        1. FROM                       
WHERE Magnitude >= 9             -- 3. WHERE       2. WHERE
ORDER BY Magnitude DESC;         -- 4. ORDER BY

Processing order

                                 -- Syntax Order | Processing Order
SELECT Country, Place, Magnitude -- 1. SELECT      3. SELECT
FROM Earthquakes                 -- 2. FROM        1. FROM                       
WHERE Magnitude >= 9             -- 3. WHERE       2. WHERE
ORDER BY Magnitude DESC;         -- 4. ORDER BY

Processing order

                                 -- Syntax Order | Processing Order
SELECT Country, Place, Magnitude -- 1. SELECT      3. SELECT
FROM Earthquakes                 -- 2. FROM        1. FROM                       
WHERE Magnitude >= 9             -- 3. WHERE       2. WHERE
ORDER BY Magnitude DESC;         -- 4. ORDER BY    4. ORDER BY

Processing errors


SELECT Country, 
       PlaceName, 
       Magnitude
FROM LargeEarthquakes            
WHERE Strength >= 9                 
ORDER BY Magnitud DESC;

Processing FROM

                        -- Processing Order
SELECT Country, 
       PlaceName, 
       Magnitude
FROM LargeEarthquakes   -- 1. FROM - table LargeEarthquakes does not exist
WHERE Strength >= 9                 
ORDER BY Magnitud DESC;

--------------------------------------------------------------------------------
-- ERROR
Invalid object name 'LargeEarthquakes'.

Processing WHERE

                        -- Processing Order
SELECT Country, 
       PlaceName, 
       Magnitude
FROM Earthquakes                              
WHERE Strength >= 9     -- 2. WHERE - column Strength does not exist 
ORDER BY Magnitud DESC;

--------------------------------------------------------------------------------
-- ERROR
Invalid column name 'Strength'.

Processing SELECT

                        -- Processing Order
SELECT Country, 
       PlaceName,       -- 3. SELECT - column PlaceName does not exist
       Magnitude 
FROM Earthquakes                              
WHERE Magnitude >= 9                 
ORDER BY Magnitud DESC;

--------------------------------------------------------------------------------
-- ERROR
Invalid column name 'PlaceName'.

Processing ORDER BY

                        -- Processing Order
SELECT Country, 
       Place, 
       Magnitude
FROM Earthquakes                              
WHERE Magnitude >= 9                 
ORDER BY Magnitud DESC;-- 4. ORDER BY - column misspelling

________________________________________________________

-- ERROR
Invalid column name 'Magnitud'.

Error free

SELECT Country, 
       Place, 
       Magnitude   
FROM Earthquakes                
WHERE Magnitude >= 9      
ORDER BY Magnitude DESC;

Country	Place	Magnitude
CL	Bio-Bio; Chile	9.5
US	Southern Alaska	9.2
ID	off the west coast of northern Sumatra	9.1
JP	near the east coast of Honshu; Japan	9.1
...	...	...

Logical processing order

1.  FROM

2.  ON

3.  JOIN

4.  WHERE

5.  GROUP BY

6.  HAVING


7.  SELECT


8.  DISTINCT

9.  ORDER BY

10. TOP

Let's practice

Improving Query Performance in SQL Server