Question

我是SQL的初学者。我正在使用查询数据库的程序，然后处理结果。默认查询为：

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;

我希望查询每个Subj_Id和每个Q_Id。我不知道有多少，不同的主题有不同的问题。我该如何改变上述问题？

Answer 1

您可以像这样重写上述查询。

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;

通过将表之间的连接与数据上的过滤器分开，可以更清楚地实现。因此，要查询整个数据库，只需从上面删除where子句......

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;

SQL：查询整个数据库

1 个答案: