| Unknown values and NULLComparison operatorsNULL comparison operators
jao# select name from birthday where birth_date = NULL;
 name 
------
(0 rows)
jao# select name from birthday where birth_date != NULL;
 name 
------
(0 rows)
jao# select name from birthday where birth_date is NULL;
       name        
-------------------
 Genghis Khan
 Pope Boniface III
(2 rows)
jao# select name from birthday where birth_date is not NULL;
      name       
-----------------
 Richard Feynman
 Donald Knuth
(2 rows)
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