sql query performance tips and experiences

[FrancisChung]

@bernhard, I'm a bit confused with the following SQL

 

(SELECT month_$i FROM demo WHERE year = $year)

Is there a missing "AND month = $i" in the WHERE clause or do you actually have 12 fields called (month_1 .... month_12).

Asking because it's not your usual normalised database table structure.

Was there a reason for going the denormalised route rather than say having a field called Month with values 1 to 12?  So your query would look like :
 

(SELECT month FROM demo WHERE year = $year and month=$i) AS month_$i

Normally you might denormalise if you have too much data or reducing the number of joins in your query dramatically improves performance.