NULL Values: Advanced

TRUE, FALSE, and NULL walk into a bar

TRUE, FALSE, and NULL walk into a bar

Category

SQL

Difficulty

advanced

Duration

26 minutes

Challenges

0 hands-on challenges

Topics covered: NULLs in window functions, NULLs in UNION operations, DISTINCT and NULL, NULL propagation, Three-valued logic (NOT IN)

Lesson Sections

1. NULLs in window functions

   Sorting and Ordering NULLS FIRST/LAST NULL in PARTITION BY Calculations and Navigation Window Aggregates + NULL LAG/LEAD with NULL

2. NULLs in UNION operations

   UNION Behavior UNION Deduplication UNION ALL Duplicates

3. DISTINCT and NULL

   DISTINCT Grouping Behavior NULL Collapsing Context Comparison NULL Behavior by Context

4. NULL propagation

   Basic Propagation Rules Arithmetic Propagation String Concatenation Control Flow Propagation CASE Propagation Function Propagation

5. Three-valued logic (NOT IN)

   SQL uses three-valued logic (TRUE, FALSE, UNKNOWN) instead of classical boolean logic. This fundamentally changes how compound conditions behave and breaks some intuitive rules. Three-Valued Logic Boolean operations in SQL don't just have TRUE and FALSE outcomes; UNKNOWN is a third possibility that propagates through logical operators. Truth Tables with UNKNOWN Common Pitfalls Three-valued logic creates counterintuitive behaviors that cause subtle bugs even for experienced developers. The NOT IN