On 6 December 2011 12:00, Adarsh Sharma <adarsh.sharma@xxxxxxxxxx> wrote: >> select * from table where lat IS NULL; > can you explain how it works or any link that explain the difference between > 2 queries. That's because of the 3-valued logic of SQL. x=NULL always evaluates to NULL, because it is unknown whether the two might be equal or not. x IS NULL checks whether x is known (NOT NULL) or not (NULL) and evaluates to TRUE or FALSE based on that. WHERE-clauses only return rows where the expression evaluates to TRUE and not rows where it evaluates to NULL or (obviously) FALSE. -- If you can't see the forest for the trees, Cut the trees and you'll see there is no forest. -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
