Unknown values and NULL
Logical operators
SQL 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 |
? |
? |
|
|
|