Question

The query:

SELECT ct_cid as level1 FROM cat_tree WHERE ct_sid=$sid_int AND ct_parent =$cid_int)
UNION (SELECT ct_cid as level2 FROM cat_tree WHERE ct_sid=$sid_int AND ct_parent IN level1)

The query is supposed to find the child nodes up to 2 levels of a tree node.
I get “You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘level1)” How can I use the results of the 1st select in the 2nd select?

Answer 1

you don’t need parens around the second SELECT statement… Also, the field names and order must match in BOTH SQL-Select statements. What you may want is something like

select 1 as LevelID, CT_CID from cat_tree where...  
UNION
select 2 as levelID, CT_CID from cat_tree where ...

Now, your other half of the question… using the results of the first… Redo the query as a sub-select in the SECOND query…

select 1 as LevelID, CT_CID 
    from cat_tree
    where ct_sid = $sid_int
      and ct_parent = $cid_int
union
select 2 as LevelID, CT_CID
    from cat_tree
    where ct_sid = $sid_int
      and ct_parent IN
              ( select ct_cid  
                     from cat_tree
                     where ct_sid = $sid_int
                       and ct_parent in $cid_int ) 

Notice the parens are only needed in the SUB-SELECT, not the UNION select. Not knowing your data, there might be an easier approach to what you are trying to get