Sql: joining two tables

Question

I have two tables called mp_Blogs and mp_BlogCategories

mp_blog has columns and values

 ItemID   ModuleID 
   3         11
   4         11
   5         13

mp_BlogCategories has column and values

 CategoryID ModuleID 
   1         11
   2         11
   3         13

I need the output like this

 ItemID   CategoryID
   3         1
   4         2
   5         3

I used this query

SELECT a.[ItemID],b.[CategoryID] 
from [mp_Blogs] a 
    join [mp_BlogCategories] b 
        on a.ModuleID=b.ModuleID

But its coming in different way

 ItemID   CategoryID
   3         1
   4         1
   3         2
   4         2
   5         3

I used groupby function also but output same as before,any mistake in query how to get my original output

SELECT a.[ItemID],b.[CategoryID] 
from [mp_Blogs] a 
    join [mp_BlogCategories] b 
        on a.ModuleID=b.ModuleID  
group by b.CategoryID,a.ItemID

Answer 1

You query is performing properly but there is problem with data in you table as you can see the table contains 11 id two time thats y its giving the result like that. its better to check out the data in table.

Answer 2

从输出看来，数据，而不是查询有问题。

Answer 3

After reading comments you are lacking for unique key in both the tables so i suggest you to add key in both the tables in order to achieve your result.

How it would be - tested in mysql working fine.

-- id column primary key in mp_blog table.

create table mp_blog(id smallint(5),ItemID smallint(5),
ModuleID smallint(5),constraint mp_blog_id primary key(id));

insert into mp_Blog values(1,3,11),(2,4,11),(3,5,13);

-- id column foreign key references to mp_blog primary key id.

create table mp_BlogCategories(id smallint(5),CategoryID smallint(5),
ModuleID smallint(5),constraint fk_mp_blogc foreign key(id) references mp_blog(id));

insert into mp_BlogCategories values(1,1,11),(2,2,11),(3,3,13);

-- finally what you are trying to achieve..

select b.itemid,bc.categoryid
from mp_blog b INNER JOIN mp_BlogCategories bc
on b.id=bc.id;


 ItemID   CategoryID
   3         1
   4         2
   5         3