| Unknown values and NULLLogical operatorsSQL uses 3-valued logic
  2-valued logic is based on boolean operations on True, False.
  3-valued logic is based on boolean operations on True, False, Unknown.
  Unknown means: The value is either True of False, but we don't know which.
 
| 
  
    |  | T | F | ? |  
    | T | T | F | ? |  
    | F | F | F | F |  
    | ? | ? | F | ? |  | 
  
    |  | T | F | ? |  
    | T | T | T | T |  
    | F | T | F | ? |  
    | ? | T | ? | ? |  |  | 
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