“ NOT IN”无法正常工作

Question

I was writing a query to find the node type from the table BST ordered by the value of the node. table, BST, had two columns N and P, where N represents the value of a node in BST, and P is the parent of N. say, BST has following records: BST Table

I successfully executed the query as follows:

SELECT n,CASE
                WHEN p IS NULL THEN 'Root'
                WHEN n IN (SELECT DISTINCT p FROM BST) THEN 'Inner'
                ELSE 'Leaf'
             END
FROM BST
ORDER BY n;

Result: Result as expected

But instead of using "IN", when I tried the same query using "NOT IN" as given below:

SELECT n,CASE
            WHEN p IS NULL THEN 'Root'
            WHEN n NOT IN (SELECT DISTINCT p FROM BST) THEN 'Leaf'
            ELSE 'Inner'
         END
FROM BST
ORDER BY n;

it didn't work as expected. Why so?

Answer 1

As @jarlh suggested, use NOT EXISTS, or when using NOT IN, be sure to exclude NULLs from your subquery like:

SELECT n,CASE
            WHEN p IS NULL THEN 'Root'
            WHEN n NOT IN (SELECT DISTINCT p FROM BST WHERE p IS NOT NULL) THEN 'Leaf'
            ELSE 'Inner'
         END
FROM BST
ORDER BY n;

Answer 2

If I were you, I'd just a NOT EXISTS. I used to always use NOT IN as a beginner. Later on I realized that you need to consider quite a bit of different factors that you never thought of at first. Use WHERE NOT EXISTS and you'll be happy fellow.

Cheers!