内部联接失败-不返回任何数据

Question

I am encountering some problems in inner joining a table and a view . The view is pageviewforum with a column forumn_no varchar2 and a column of totalcount which is the count of total pageviews in each forum.

Another table is forum : forum_no char(6), forumname varchar2(50)

I want to output the totalcount with the forum_no and its forum name but it doesn't work.

select forum.forumname, totalcount from pageviewforum 
inner join forum on pageviewforum.forum_no= forum.forum_no; 

The above query returns the error message of no data. I tried to see what comes out of

select forum.forumname, totalcount from pageviewforum 
inner join forum on pageviewforum.forum_no > forum.forum_no;

and data comes out. I tried to use like but it says no data as well. What can I do?

Answer 1

Since you are joining a varchar2 column of indeterminate size with a char(6) column, it may be that you are running into differences because of that. If those columns hold a number, why not use a numeric type (like INTEGER) to hold the values? It will be more reliable in the long run. Generally, joining columns work most reliably when they're of the same type. When they're of different types, you have to worry about whether the two types are compatible enough and whether they compare sensibly.

Hypothesis : In particular, a CHAR(6) column is blank-padded to full length; unless the DBMS strips those trailing blanks off, you may never get a CHAR(n) and a VARCHAR(n) to compare equal unless all n characters are in use. The > comparison works because the blanks matter for equality, but the greater than or less than can still come up with an answer.

Observations:

• Column types in a database do matter.
• Consistency is important.