Question

I was given a query to explain. Could someone please explain it to me:

select j.ip_num from 
jobs j, address a
where j.jobtype='C' and
a.sel_code(+)='H' and 
j.ip_num=a.ip_num and
a.ip_num is null order by a.ip_num

Answer 1

The query is joining the 2 tables jobs, and address. These tables are joining on the field ip_num but you are looking for the records that exist in the jobs table but do not exist in the address table.

This is a LEFT OUTER JOIN. This query could also be written

SELECT j.ip_num 
FROM jobs j
LEFT OUTER JOIN address a
    ON j.ip_num=a.ip_num
WHERE j.jobtype='C' AND
    a.sel_code(+)='H' AND 
    a.ip_num is null 
ORDER BY a.ip_num

It might be useful to see a visual picture joins http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html