SQL multiple Select ... where id in and a DISTINCT

Question

I am running a query with multiple "where id in" clauses on a table in a MySQL db. I put DISTINCT in each query to try to cut down the number of results. Still locked up my MySQL table. Any glitches I don't know about, or should I try getting rid of some of the DISTICTs? Only the first DISTINCT is necessary.

   SELECT DISTINCT id, name 
   FROM TP_Test_Info 
   WHERE id IN (SELECT DISTINCT test_id 
                FROM TP_Test_Sections 
                WHERE id IN (SELECT DISTINCT section_id 
                             FROM TP_Test_Questions 
                             WHERE id IN (SELECT DISTINCT question_id 
                                          FROM TP_Student_Answers 
                                          WHERE student_id = 751)))

The last table TP_Student_Answers has about 32,000 entries. The rest are much smaller.

I realized that I should have indexed some of the rows in the big table. Spaced it. Any other problems stick out?

Answer 1

You should be using joins and consider creating indexes at least on foreign keys

Try the following

SELECT DISTINCT TP_Test_Info.id, TP_Test_Info.name
FROM TP_Test_Info
JOIN TP_Test_Sections 
  ON TP_Test_Sections.test_id = TP_Test_Info.id
JOIN TP_Test_Questions 
  ON TP_Test_Questions.section_id = TP_Test_Sections.id
JOIN TP_Student_Answers 
  ON TP_Student_Answers.question_id = TP_Test_Questions.id
WHERE TP_Student_Answers.student_id = 751