raducu gheorghe's Profile

The theory (by theory I mean SQL Standard) says that WHERE restricts the result set before returning rows and HAVING restricts the result set after bringing all the rows. So WHERE is faster. On SQL Standard compliant DBMSs in this regard, only use HAVING where you cannot put the condition on a WHERE (like computed columns in some RDBMSs.)

You can just see the execution plan for both and check for yourself, nothing will beat that (measurement for your specific query in your specific environment with your data.)

Favor using CASE expressions under the following circumstances:





• 
  

   when
  data needs to be converted from one type to another for display and no
  function exists to accomplish the task
  
  
  

• 
  

   when
  a summary row needs to be created from detail data
  
  
  

• 
  

  when
  conditional processing needs to be executed to determine results
  
  
  

• 
  

  when
  using UNION to “glue together” different subsets of a single table