Question

I am a complete beginner in SQL. I am using a program that queries a database, and then processes the results. The default query is:

SELECT * 
FROM data, 
     questions, 
     users 
where users.U_Id = data.Subj_Id 
  and data.Subj_Id between 1 and 10 
  and data.Q_Id = questions.Q_Id 
  and questions.Q_Id between 1 and 10 
order by Subj_Id;

I’d like it to query every Subj_Id and every Q_Id. I do not know how many there are of either, and different subjects have different numbers of questions. How should I alter the above query?

Answer 1

You can rewrite the above query like this.

select * 
from 
    data 
        inner join 
    users on users.U_Id = data.Subj_Id 
        inner join 
    questions on data.Q_Id = questions.Q_Id 
where data.Subj_Id between 1 and 10 
and questions.Q_Id between 1 and 10 
order by Subj_Id; 

This makes it clearer by separating the joins between tables from the filters on the data.
So to query the entire database, you just remove the where clause from the above…

select * 
from 
    data 
        inner join 
    users on users.U_Id = data.Subj_Id 
        inner join 
    questions on data.Q_Id = questions.Q_Id 
order by Subj_Id;