NULL：第3の真理値

SELECT NULL = NULL;       -- NULL (not true!)
SELECT NULL = 0;          -- NULL
SELECT NULL = '';        -- NULL
SELECT NULL <> 1;         -- NULL

SELECT email FROM users WHERE deleted_at IS NULL;       -- active users
SELECT email FROM users WHERE deleted_at IS NOT NULL;   -- deleted users

-- Suppose age is NULL for some users:
SELECT * FROM users WHERE age >= 18;
-- Users with NULL age are NOT returned, even though we
-- don't actually know they're under 18.

SELECT * FROM users
WHERE name NOT IN (SELECT excluded_name FROM blocklist);
-- If blocklist has a NULL row, the whole NOT IN evaluates to NULL
-- → no rows returned

-- Safer:
SELECT * FROM users u
WHERE NOT EXISTS (
  SELECT 1 FROM blocklist b WHERE b.excluded_name = u.name
);

SELECT id, COALESCE(full_name, email, 'Anonymous') AS display_name
FROM users;

-- If full_name is NULL, use email; if that's NULL too, use 'Anonymous'.

SELECT NULLIF(price, 0) FROM products;
-- Returns NULL when price is 0, useful for AVG() that should ignore zeros.

-- Suppose:  prices = [10, 20, NULL, 30]
SELECT COUNT(*)     FROM products;  -- 4 (counts rows)
SELECT COUNT(price) FROM products;  -- 3 (counts non-NULL prices)
SELECT AVG(price)   FROM products;  -- 20  (60 / 3, not 60 / 4)
SELECT SUM(price)   FROM products;  -- 60

SELECT * FROM users ORDER BY last_login_at DESC NULLS LAST;