How do you find rows with NULL values in a specific column, and why can't you use = NULL?

NULL in SQL is not a value — it's the absence of a value. Because of this, the equality operator = doesn't work:

-- This returns NO rows, even if shipped_at has NULLs
SELECT * FROM orders WHERE shipped_at = NULL;   -- WRONG

-- Correct
SELECT * FROM orders WHERE shipped_at IS NULL;
SELECT * FROM orders WHERE shipped_at IS NOT NULL;

QA uses:

• Verify that a required column isn't NULL after an INSERT:

  SELECT COUNT(*) FROM orders WHERE customer_id IS NULL;
  -- Expect 0
  

• Find records that should have been populated but weren't (e.g., order confirmation emails not sent):

  SELECT order_id, created_at FROM orders
  WHERE  confirmation_sent_at IS NULL
    AND  status = 'confirmed';
  

NULL also propagates through arithmetic: any expression involving NULL returns NULL. Be aware when summing or averaging columns that may have NULLs — use COALESCE(col, 0) to substitute a default.