SQL：查詢整個數據庫

Question

我是一個完整的SQL初學者。我正在使用查詢數據庫的程序，然後處理結果。默認查詢是：

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; 

我想它來查詢每個Subj_Id每Q_Id。我不知道有多少，不同的科目有不同數量的問題。我應該如何改變上述查詢？

Answer 1

您可以像這樣重寫上述查詢。

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; 

這使得通過從數據上的過濾器中分離表格之間的連接更加清楚。 所以要查詢整個數據庫，你只要從上面取下where條款等

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;