Part 20: Handling NULL Values: IS NULL & Three-Valued Logic

NULL in SQL represents the absence of data or an unknown value—it does NOT equal zero 0, false, or an empty string ''.

1. Three-Valued Logic (3VL)

SQL boolean expressions evaluate to one of three states: - TRUE - FALSE - UNKNOWN (Result of any comparison with NULL)

-- ❌ NEVER DO THIS: Returns UNKNOWN (0 rows returned)
SELECT * FROM users WHERE phone = NULL;

--  CORRECT:
SELECT * FROM users WHERE phone IS NULL;
SELECT * FROM users WHERE phone IS NOT NULL;

Key Takeaways

• NULL is not a value; it is a marker of unknown state.
• Always use IS NULL and IS NOT NULL for comparisons.

In Part 21, we enter Module 3 to master Sorting Results with ORDER BY!