Question

I have a simple SQL query:

SELECT  
       columnA, columnB, columnC...  
 FROM  
       (SELECT    
             columnA, columnB, columnC...  
        FROM   
              SomeTable) Table1  
  WHERE NOT EXISTS  
        (SELECT   
               columnA  
          FROM  
               SomeOtherTable st  
          WHERE  
               st.columnB = Table1.columnB)

Can anyone give me a suggestion how to rewrite this query for better performance? I mean to include the WHERE NOT EXISTS clause in Table1.

Answer 1

You can use this:

select Table1.*
from (select * from SomeTable) Table1
left outer join SomeOtherTable sot
    on Table1.columnB = sot.columnB
where sot.columnB is null;

For the performance it is important to have indexes on columnB on both tables.