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Fun with three-valued logic

I thought I was pretty used to SQL's three-valued logic by now, but this still caused me a minute of scratching my head: 

# select count(*) from _t;
 count
-------
  1306
(1 row)

# select count(*) from _t2;
 count
-------
 19497
(1 row)
Both _t and _t2 are temporary tables of a single column I created with SELECT DISTINCT. 
# select count(*) from _t where userhash in (select userhash from _t2);
 count
-------
   982
(1 row)

# select count(*) from _t where userhash not in (select userhash from _t2);
 count
-------
     0
(1 row)

Hmm, 982 + 0 != 1306...

Turns out there was a null in _t2; X in {set containing null} evaluates to null, not false, and negating null still gives null. (The rule of thumb is, any operation on null is still null.)

.................

I'm giving a tutorial on Advanced Databases with SQLAlchemy at PyCon in February. Feel free to let me know if there is anything you'd like me to cover specifically.

Comments

Anonymous said…
Thank you!!! you just solved my long standing django bug with select_related and null relations!

http://code.djangoproject.com/ticket/3149
9:14 PM
Jonathan Ellis said…
Cool, glad I could help. :)
10:11 PM
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