| Unknown values and NULLComparison operatorsBut what about NULL = NULL?Each occurrence of NULL represents a different unknown.
E.g., consider this table:
 
birthday
 
  
    | name | birth_date |  Richard Feynman | May 11, 1918 | 
   Genghis Khan | NULL | 
   Donald Knuth | January 10, 1938 | 
   Pope Boniface III | NULL | 
   
 
  Pope Boniface III and Genghis Khan definitely have birthdates.
  But those dates are unknown.
  The birthdates are probably different, since they lived 600 years apart.
 SQL does group different unknowns together sometimes, but in almost
all situations, different NULLs are different.
 
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