Stage 6: Null Handling

Stage Objective

Add support for NULL values, nullable column definitions, and IS NULL / IS NOT NULL conditions.

Conceptual Algorithms

• Extend the CREATE TABLE parser to accept NULL or NOT NULL constraints (defaulting to nullable).
• Modify the serialization format. Since we have fixed-size and varlen pages, NULL can be represented using a null-bitmap at the start of the row header. 1 bit per column.
• Update the SQL Lexer/Parser to recognize NULL as a value literal.
• Add support for IS NULL and IS NOT NULL in the WHERE clause parser and executor logic.
• Ensure aggregate functions ignore NULL values correctly (except COUNT(*)).

Implementation Checklist

• Add NULL, IS, NOT tokens.
• Support IS NULL and IS NOT NULL in the AST.
• Implement a null-bitmap in the tuple serialization layer.
• Update insert logic to handle null constraints.
• Update filter logic in Sequential/Index Scans.

Expected Contract

When inserting a row missing nullable columns, it should be stored as NULL. SELECT ... WHERE col IS NULL should return the correct rows. NOT NULL constraints should raise an error when violated.