SQL DDL

sqlite3 test_guru.db
sqlite> CREATE TABLE categories(
   ...> id INTEGER PRIMARY KEY AUTOINCREMENT,
   ...> title TEXT
   ...> );
sqlite> CREATE TABLE tests(
   ...> id INTEGER PRIMARY KEY AUTOINCREMENT,
   ...> title TEXT,
   ...> level INTEGER,
   ...> category_id INTEGER,
   ...> FOREIGN KEY(category_id) REFERENCES categories(id)
   ...> );
sqlite> CREATE TABLE questions(
   ...> body TEST,
   ...> test_id INTEGER,
   ...> FOREIGN KEY(test_id) REFERENCES tests(id)
   ...> );
sqlite> INSERT INTO categories(title) VALUES ('world');
sqlite> INSERT INTO categories(title) VALUES ('cars');
sqlite> INSERT INTO categories(title) VALUES ('cats');
sqlite> INSERT INTO tests(title, level, category_id) VALUES ('test1', 0, 2);
sqlite> INSERT INTO tests(title, level, category_id) VALUES ('test2', 1, 2);
sqlite> INSERT INTO tests(title, level, category_id) VALUES ('test3', 2, 2);
sqlite> INSERT INTO tests(title, level, category_id) VALUES ('test1', 0, 1);
sqlite> INSERT INTO tests(title, level, category_id) VALUES ('test1', 0, 3);
sqlite> INSERT INTO questions(body, test_id) VALUES ('question1', 1);
sqlite> INSERT INTO questions(body, test_id) VALUES ('question2', 1);
sqlite> INSERT INTO questions(body, test_id) VALUES ('question1', 2);
sqlite> INSERT INTO questions(body, test_id) VALUES ('question2', 2);
sqlite> INSERT INTO questions(body, test_id) VALUES ('question3', 2);
sqlite> SELECT *
   ...> FROM tests
   ...> WHERE level = 2 OR level = 3;
id          title       level       category_id
----------  ----------  ----------  -----------
3           test3       2           2
sqlite> SELECT *
   ...> FROM questions
   ...> WHERE test_id = 2;
body        test_id
----------  ----------
question1   2
question2   2
question3   2
sqlite> update tests set title = 'modify_title', level = 3 where id = 4;
sqlite> DELETE
   ...> FROM questions
   ...> WHERE test_id = 2;
sqlite> SELECT tests.title, categories.title
   ...> FROM tests
   ...> JOIN categories
   ...> ON tests.category_id = categories.id;
title       title
----------  ----------
test1       cars
test2       cars
test3       cars
modify_tit  world
test1       cats
sqlite> SELECT questions.body, tests.title
   ...> FROM questions
   ...> JOIN tests
   ...> ON questions.test_id = tests.id;
body        title
----------  ----------
question1   test1
question2   test1

здесь стоит сделать alias на выборку столбцов, они у тебя с одним именем и не понятно где какой