Question

I am trying to make a query where 3 tables are being used. The first, table1, is used to store id values for table2 and table3 as well as some other data. In some cases there are no values stored in either of table2 or table3, some cases involve one or the other, and some involve both.

The issue is that when there are not values in either of table2 or table3, SQL attempts to look up NULL values as follows:

SELECT table1.id, table2value, table3value
FROM table1, table2, table3
WHERE table1value1 = table2.id AND table1value2 = table3.id;

So in this query if table1value1 or table1value2 are NULL, the query will not work as it is obviously not a valid link. So I am wondering how I would go about finding only the values that exist.

Answer 1

Use INNER JOIN instead. It joins the tables together with the condition of existing data in all tables. Else that row won’t be returned.

SELECT table1.id, table2.value, table3.value
FROM table1
INNER JOIN table2
ON table1.value1 = table2.id
INNER JOIN table3
ON table1.value2 = table3.id;

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But if you require for only one of these 2 rows to have existing data, you can do like this:

SELECT table1.id, table2.value, table3.value
FROM table1
LEFT JOIN table2
ON table1.value1 = table2.id
LEFT JOIN table3
ON table1.value2 = table3.id;
WHERE table2.id IS NOT NULL OR table3.id IS NOT NULL