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Use Joins instead of Subqueries

  
      

      

      
      

      

      Capitals
      

      

      Country         
      Capital         
      

      

      India
      New Delhi
      

      

      USA
      Washington
      

      

      France
      Paris
      

      

      		
      
      
      
      

      

      Continents
      

      

      Country     
      Continent   
      

      

      India
      Asia
      

      

      USA
      N.America
      

      

      Italy
      Europe
      

      

      		
      

      

    
 ----  Low Performance using Subqueries

      SELECT  * 
      FROM    Capitals cp
      WHERE   EXISTS (SELECT *
      FROM    Continents cn
      WHERE   cp.Country = cn.Country)


      
 ----  High Performance using Joins

      SELECT      Capitals.*
      FROM        Capitals
      INNER JOIN  Continents
      ON          Capitals.Country=Continents.Country
      

 ----  both Queries will give the same output but Performance is different, if the data is tooooo big 

      

      
Output

      

      Country   
      Capital 
      

      

      India
      New Delhi 
      

      

      USA       
      Washington