Question

I have two tables in my database:

test1 which looks like:

ID   pubmed_ID
-------------------
1    22
2    22

test2 looks like:

antigen    pubmed_ID
------------------
Hello      22
Bye        22

when i use the following select statment:

select * 
  from test1, test2 
 where test1.pubmed_ID = test2.pubmed_ID;

I get:

ID pubmed_ID antigen pubmed_ID
--------------------------------
1  22        Hello   22   
2  22        Hello   22   
1  22        Bye     22       
2  22        Bye     22

Why have the antigens been duplicated? When they only exist once in the test2 table?

The primary keys are as follows test1 = “ID” column and for test 2 both the “antigen” and “pubmed_ID” form a compund primary key

Am i missing something simple here?

Answer 1

Because both are matches. Which do you expect it to pick?

All the rows have the same pubmed_ID. Thus, each row of test1 matches each row of test2, so there are four (2*2) rows in the joined table.