Question

I have a database with two tables that are similar to:

table1
------
Id : long, auto increment
Title : string(50)
ParentId : long

table2
------
Id : long, auto increment
FirstName : string(20)
LastName : string(30)
Zip : string(5)

table2 has a one-to-many relationship with table1 where many includes zero.

I also have the following query (that works correctly, so ignore typos an the like, it is an example):

SELECT t1.Id AS tid, t1.Title, t2.Id AS oid, t2.FirstName, t2.LastName
    FROM table t1
    INNER JOIN table2 t2 ON t1.ParentId = t2.Id
    WHERE t2.Id IN
        (SELECT Id FROM table2
         WHERE Zip IN ('zip1', 'zip2', 'etc'))
    ORDER BY t2.Id DESC

The query finds all items in table1 that belong to a person in table2, where the person is in one of the listed zip codes.

The problem I have now is: I want to show all the users (with their items if available) in the listed zip codes, not just the ones with items.

So, I am wondering, should I just do something simple with a lot more queries, like:

SELECT Id AS oid, FirstName, LastName FROM table2 WHERE Zip in ('zip1', 'zip2', 'etc')
foreach(result) {
    SELECT Id AS tid, Title FROM table2 WHERE ParentId = oid
}

Or should I come up with a more elaborate single SQL statement? And if so, can I get a little help? Thanks!

Answer 1

I agree with (and have upvoted) @Lee D and @Bueller. However, I generally advocate LEFT OUTER JOINS, because I find it easier to conceptualized what’s going on with them, particularly when you are joining three or more tables. Consider it like so:

Start with what you know you want in the final result set:

FROM table2 t2

and then add in the “optional” data.

FROM table2 t2
 left outer join table1 t1
  on t1.ParentId = t2.Id

Whether or not matches are found, whatever gets selected from table2 will always appears.