Thread: no cond returns diff res than cond or !cond
Good day, feline=> select version(); version ---------------------------------------------------------------PostgreSQL 7.0.2 on i686-pc-linux-gnu, compiled by gcc 2.95.2 (1 row) feline=> select count(*) from produkte;count -------36405 (1 row) feline=> select count(*) from produkte where artnummer != NULL or artnummer = NULL;count -------36347 (1 row) cenes_test=> select version(); version -------------------------------------------------------------PostgreSQL 7.1.3 on i686-pc-linux-gnu, compiled by GCC 2.96 (1 row) cenes_test=> select count(*) from produkte;count -------36384 (1 row) cenes_test=> select count(*) from produkte where artnummer = NULL or artnummer != NULL;count -------36326 (1 row) 2nd query 58 less than first. bug? Markus Bertheau
On 12 Nov 2001, Markus Bertheau wrote: > cenes_test=> select version(); > version > ------------------------------------------------------------- > PostgreSQL 7.1.3 on i686-pc-linux-gnu, compiled by GCC 2.96 > (1 row) > > cenes_test=> select count(*) from produkte; > count > ------- > 36384 > (1 row) > > cenes_test=> select count(*) from produkte where artnummer = NULL or > artnummer != NULL; > count > ------- > 36326 > (1 row) > > 2nd query 58 less than first. > > bug? Probably not. You don't want to compare to NULL (although there is a hack which makes an explicit = NULL into the correct comparison), you want to use IS NULL and IS NOT NULL. Comparing to NULL returns an unknown (neither true nor false) and negating that comparison still returns unknown. The hack is in there for compatibility with an MS Access feature, and I think is optional in 7.2, and turns = NULL into IS NULL, there's no equivalent hack for != NULL AFAIK.
Hi Markus, AFAIK the SQL standard says for querying columns to be NULL resp. NOT NULL select count(*) from produkte where artnummer is NULL or artnummer is NOT NULL ; Please try and let me know about your results. Regards, Christoph > > Good day, > > feline=> select version(); > version > --------------------------------------------------------------- > PostgreSQL 7.0.2 on i686-pc-linux-gnu, compiled by gcc 2.95.2 > (1 row) > > feline=> select count(*) from produkte; > count > ------- > 36405 > (1 row) > > feline=> select count(*) from produkte where artnummer != NULL or > artnummer = NULL; > count > ------- > 36347 > (1 row) > > cenes_test=> select version(); > version > ------------------------------------------------------------- > PostgreSQL 7.1.3 on i686-pc-linux-gnu, compiled by GCC 2.96 > (1 row) > > cenes_test=> select count(*) from produkte; > count > ------- > 36384 > (1 row) > > cenes_test=> select count(*) from produkte where artnummer = NULL or > artnummer != NULL; > count > ------- > 36326 > (1 row) > > 2nd query 58 less than first. > > bug? > > Markus Bertheau > > > > > ---------------------------(end of broadcast)--------------------------- > TIP 3: if posting/reading through Usenet, please send an appropriate > subscribe-nomail command to majordomo@postgresql.org so that your > message can get through to the mailing list cleanly >