⇒ It is possible for tuples to have a NULL value for some of their attributes.
⇒ NULL signifies:
⇒ An unknown value or missing data, or
⇒ A value that does not exist.
Ex
⇒ Any arithmetic expression (+, -, *, /) involving NULL results in NULL.
⇒ Example: 10 + NULL = NULL
⇒ Example: NULL * 20 = NULL
Aggregate Functions:⇒ Aggregate functions ignore NULL values except for count.
⇒ Functions like avg, min, max, and sum exclude NULL values from their calculations but still count the total number of values.
Duplication and Grouping:⇒ For duplication elimination and grouping, NULL is treated like any other value. Two NULL values are assumed to be the same.
Comparisons:⇒ Comparisons (<, <=, >, >=, =, ≠) involving NULL evaluate to a special value called unknown (as in SQL).
⇒ Example: 5 = NULL, NULL > 5, 5 > NULL, NULL = NULL all result in unknown.
⇒ Because the outcome of these comparisons is neither true nor false, it is considered unknown.
Three-Valued Logic:⇒ Boolean expressions involving AND, OR, and NOT use three-valued logic: true (1), false (0), and unknown.
OR Operation:⇒ (unknown OR true) = true
⇒ (unknown OR false) = unknown
⇒ (unknown OR unknown) = unknown
AND Operation:⇒ (true AND unknown) = unknown
⇒ (false AND unknown) = false
⇒ (unknown AND unknown) = unknown
NOT Operation:⇒ (NOT unknown) = unknown
⇒ The result of a SELECT predicate is treated as false if it evaluates to unknown.
