SELECT COUNT(DISTINCT normalized_email) AS unique_users
FROM (
    SELECT
      CASE
        WHEN email ILIKE '%@gmail.com' THEN
          REGEXP_REPLACE(
            SPLIT_PART(SPLIT_PART(LOWER(TRIM(email)), '+', 1), '@', 1),
            '\.', '', 'g'
          ) || '@gmail.com'
        ELSE
          LOWER(REPLACE(TRIM(email), ' ', ''))
      END AS normalized_email
    FROM users
) AS cleaned;

SELECT COUNT(*) > 0
FROM orders
WHERE user_id = 42;

SELECT EXISTS (
    SELECT 1
    FROM orders
    WHERE user_id = 42
);

CREATE INDEX idx_orders_user_id ON orders(user_id);

SELECT *
FROM users
WHERE id NOT IN (
    SELECT user_id
    FROM banned_users
);

SELECT u.*
FROM users AS u
WHERE NOT EXISTS (
    SELECT 1
    FROM banned_users AS b
    WHERE b.user_id = u.id
);

SELECT NULL = NULL;

NULL

WHERE a = b
   OR (a IS NULL AND b IS NULL)

a IS NOT DISTINCT FROM b

SELECT NULL IS NOT DISTINCT FROM NULL; -- true
SELECT 10   IS NOT DISTINCT FROM 10;   -- true
SELECT 10   IS NOT DISTINCT FROM NULL; -- false

a IS DISTINCT FROM b

SELECT *
FROM old_data o
JOIN new_data n USING (id)
WHERE o.email IS DISTINCT FROM n.email;

o.email <> n.email

SELECT country, city, SUM(revenue)
FROM sales
GROUP BY country, city

UNION ALL

SELECT country, NULL, SUM(revenue)
FROM sales
GROUP BY country

UNION ALL

SELECT NULL, NULL, SUM(revenue)
FROM sales;

SELECT
    country,
    city,
    SUM(revenue) AS revenue
FROM sales
GROUP BY GROUPING SETS (
    (country, city),
    (country),
    ()
);

GROUPING(country)
GROUPING(city)

ROLLUP(...)
CUBE(...)