MySQL 5.7

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CREATE TABLE table_content (
  `id` INTEGER,
  `section` VARCHAR(1),
  `content_id` INTEGER
);

INSERT INTO table_content
  (`id`, `section`, `content_id`)
VALUES
  ('1', 'A', '15'),
  ('2', 'B', '25'),
  ('3', 'A', '9');
CREATE TABLE table_a (
  `id` INTEGER,
  `name` VARCHAR(7),
  `message` VARCHAR(14)
);

INSERT INTO table_a
  (`id`, `name`, `message`)
VALUES
  ('9', 'John', 'Hello Everyone'),
  ('15', 'Smita', 'Hi'),
  ('17', 'Vinayak', 'How are you?');
CREATE TABLE table_b (
  `id` INTEGER,
  `label` VARCHAR(5),
  `description` VARCHAR(2)
);

INSERT INTO table_b
  (`id`, `label`, `description`)
VALUES
  ('1', 'David', 'D1'),
  ('5', 'Alia', 'D2'),
  ('25', 'Vinay', 'D3');
SELECT tc.`id`, tc. `section` ,t1.`message`
FROM table_content tc JOIN (SELECT `id`, `name`, `message` FROM table_a UNION SELECT `id`, `label`, `description` FROM table_b) t1 
ON tc.`content_id` = t1.`id`


id
section
message


1
A
Hi

2
B
D3

3
A
Hello Everyone