Question

I have a database with 2 tables:

Table 1:

CREATE TABLE IF NOT EXISTS `sales` (
  `sale_id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) NOT NULL,
  `sale_total` int(11) NOT NULL,
  `sale_date` date NOT NULL,
  `sale_status` int(11) NOT NULL,
  PRIMARY KEY (`sale_id`)
) ;

Table 2:

CREATE TABLE IF NOT EXISTS `users` (
  `user_id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(200) NOT NULL,
  `lastname` varchar(200) NOT NULL,
  `mail` varchar(200) NOT NULL,
  PRIMARY KEY (`user_id`)
);

I need the following query optimized, so that it does not use subqueries. I this this could be accomplished using joins, but I don’t know exactly how.

SELECT name, lastname, mail
FROM users
WHERE user_id IN (
   SELECT user_id
   FROM sales
   WHERE sale_date < '2009-01-01'
   AND sale_total >100
   AND sale_status =4
)
AND user_id NOT IN (
    SELECT user_id
    FROM sales
    WHERE sale_date >= '2009-01-01'
)

Answer 1

Use joins to replace IN subqueries, null-left-joins to replace NOT IN, and GROUP BY to return only one row for each user:

SELECT users.name, users.lastname, users.mail
FROM users
JOIN sales AS s0 ON s0.user_id=users.user_id
LEFT JOIN sales AS s1 ON s1.user_id=users.user_id AND sale_date>='2009-01-01'
WHERE s1.sale_id IS NULL
AND s0.sale_date < '2009-01-01' AND s0.sale_total>100 AND s0.sale_status=4
GROUP BY users.user_id