NULL represents missing or unknown data. Understanding NULL behavior is crucial for accurate queries.

Check for NULL

SELECT * FROM customers WHERE phone IS NULL;

Check for NOT NULL

SELECT * FROM customers WHERE email IS NOT NULL;

NULL in Comparisons

-- This WON'T work!

SELECT * FROM customers WHERE phone = NULL;

-- Use IS NULL instead

SELECT * FROM customers WHERE phone IS NULL;

COALESCE - Replace NULL

SELECT name, COALESCE(phone, 'No phone') as phone FROM customers;

IFNULL (MySQL)

SELECT name, IFNULL(phone, 'N/A') as phone FROM customers;

NULLIF

SELECT NULLIF(discount, 0) as discount FROM products;

COUNT and NULL

SELECT COUNT(*) as total, COUNT(phone) as with_phone FROM customers;

Handle NULL with AI2sql

AI2sql correctly handles NULL in generated queries.