Uses for OUTER JOINs
Because an OUTER JOIN lets you see not only the matched rows but also the unmatched ones, it’s great for finding out which, if any, rows in one table do not have a matching related row in another table. It also helps you find rows that have matches on a few rows but not on all. In addition, it’s useful for creating input to a report where you want to show all categories (regardless of whether matching rows exist in other tables) or all customers (regardless of whether a customer has placed an order). Following is a small sample of the kinds of requests you can solve with an OUTER JOIN.
Find Missing Values
Sometimes you just want to find what’s missing. You do so by using an OUTER JOIN with a test for Null. Here are some ...
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