MySQL 8.0

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create table table1(id integer, num integer);
insert into table1(id, num) values(1, 3),(2, 5),(3, 6);
SELECT * FROM table1;
create table table2(uni_id integer, id integer);
insert into table2(uni_id, id) values(1, 1),(2, 2),(3, 2),(4, 2),(5, 3);
SELECT * FROM table2;
create table table3(uni_id integer);
insert into table3(uni_id) values(1),(2),(3),(5);
SELECT * FROM table3;

id
num

1
3

2
5

3
6

uni_id
id

1
1

2
2

3
2

4
2

5
3

uni_id

1

2

3

5

-- Критерий поиска
SET @number := 5;
-- выборка из таблицы 1
SELECT t1.id
FROM table1 t1
WHERE t1.num = @number;

id

2

-- выборка соотв. записей из таблицы 2
SELECT t2.uni_id
FROM table1 t1
JOIN table2 t2 USING (id)
WHERE t1.num = @number;

uni_id

2

3

4

-- выборка нужных данных из таблицы 3
SELECT t3.uni_id
FROM table1 t1
JOIN table2 t2 USING (id)
JOIN table3 t3 USING (uni_id)
WHERE t1.num = @number
ORDER BY t3.uni_id DESC LIMIT 1;

uni_id

3