SELECT *

FROM main

JOIN (

    SELECT '1' AS source, * FROM slave1

    UNION ALL

    SELECT '2', * FROM slave2

    UNION ALL

    ....

    ) AS slave ON ...

CREATE TABLE `Users`(`Id` INT, `UserName` TEXT);

INSERT `Users`(`Id`, `UserName`) VALUES (1, 'Vasily Pupkin');

 

CREATE TABLE `Categories`(`Id` INT, `CategoryName` TEXT);

INSERT `Categories`(`Id`, `CategoryName`) VALUES (1, 'PHP Developers'), (2, 'English Speakers');

 

CREATE TABLE `User2Categorу`(`UserId` INT, `CategoryId` INT, `SkillLevel` INT);

INSERT `User2Categorу`(`UserId`, `CategoryId`, `SkillLevel`) VALUES (1, 1, 4), (1, 2, 3);

CREATE TABLE `users`(`id` INT, `name` TEXT);

INSERT `users`(`id`, `name`) VALUES (1, 'Vasily Pupkin');

CREATE TABLE `values_1`(`user_id` INT, `value_1` INT);

INSERT `values_1`(`user_id`, `value_1`) VALUES (1, 1);

 

CREATE TABLE `values_2`(`user_id` INT, `value_2` INT);

INSERT `values_2`(`user_id`, `value_2`) VALUES (1, 2);

SELECT `users`.`name`, `values_1`.`value_1`, `values_2`.`value_2` FROM `users` INNER JOIN `values_1`, `values_2`;

CREATE TABLE `values`(`user_id` INT, `value` INT, `index` INT);

INSERT `values`(`user_id`, `value`, `index`) VALUES (1, 1, 1), (1, 2, 2);