Rewriting Subquery Within Select Due to Oracle Materialized View Limitation

Question

Oracle poses a limitation on using a subquery within the select clause when creating a materialized view. When you do so, you receive the error "ORA-22818: subquery expressions not allowed here".

Due to this limitation, I've been struggling to rewrite the query and move the subquery out of the select clause. The query is recursively building a path using parent/child relationships, and I'm trying to also indicate if a particular category is a leaf category by joining the table back to itself and seeing if the record has a child.

SELECT A.PRODUCTCATEGORYID, A.PARENTCATEGORYID, SYS_CONNECT_BY_PATH(A.LABEL, ':') "PATH", 
(
        SELECT CASE WHEN MAX(PRODUCTCATEGORYID) IS NOT NULL THEN 0 ELSE 1 END 
        FROM PRODUCT_CATEGORY
        WHERE parentcategoryid = A.PRODUCTCATEGORYID
) as "LEAF"
FROM PRODUCT_CATEGORY A
CONNECT BY PRIOR A.PRODUCTCATEGORYID = A.PARENTCATEGORYID
START WITH A.PARENTCATEGORYID IS NULL;

Can anyone point me in the right direction of how I should go about rewriting this so the subquery is not part of the select clause?

Thanks in advance.

Answer 1

You can use the CONNECT_BY_ISLEAF pseudocolumn, which returns 1 if current node is a leaf, and 0 otherwise, so your query should be rewritten like this:

SELECT A.PRODUCTCATEGORYID, A.PARENTCATEGORYID, SYS_CONNECT_BY_PATH(A.LABEL, ':') "PATH", 
  CONNECT_BY_ISLEAF as "LEAF"
FROM PRODUCT_CATEGORY A
CONNECT BY PRIOR A.PRODUCTCATEGORYID = A.PARENTCATEGORYID
START WITH A.PARENTCATEGORYID IS NULL;

Read more in Oracle's documentation: CONNECT_BY_ISLEAF pseudocolumn