вам нужно создать динамическое состояние:
DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `search_equipment`(IN equip VARCHAR(100), IN category VARCHAR(255))
BEGIN
SET @s =
CONCAT('SELECT *
FROM Equipment
WHERE e_description
LIKE \'%',equip,'%\'
AND e_type IN (',category,')');
PREPARE stmt from @s;
EXECUTE stmt;
DEALLOCATE PREPARE stmt3;
END$$