Question

This must be easier than I am making it, but I am going crazy here.
What I want to do is create a view from three tables, like such:

SELECT tableA.name, tableB.id, tableB.categoryID, tableC.categoryParentID
FROM tableB, tableA where tableA.id=tableB.id JOIN tableC on
tableC.categoryID = tableB.categoryID;

Simple enough. However, what I want to accomplish is that in the case where tableB.categoryID is NULL, I want tableC.categoryParentID to show up as a NULL value rather than as the categoryParentID value.

So far with my results I either can get the table to show all the rows where tableB.categoryID IS not NULL, and exclude the rows that have a null value for categoryID entirely – I don’t want to do this – OR I get some weird variation that includes every row for categoryParentID from tableC for every row from tableB – which gives me upwards of 3,000,000 rows, and is also incorrect.

If this is unclear, I can explain more.

Thanks in advance.

Answer 1

If I’m understanding you correctly, what you want is:

SELECT tableA.name,
       tableB.id,
       tableB.categoryID,
       tableC.categoryParentID
  FROM tableA
  JOIN tableB
    ON tableB.id = tableA.id
  LEFT
 OUTER
  JOIN tableC
    ON tableC.categoryID = tableB.categoryID
;

(See the “left outer join” section of the Wikipedia article on SQL joins.)