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 (concepts: sqlDistinct)

   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