Question

I currently have a table which has these columns:

id (INT)
parent_id (INT)
col0
col1
col2

As an example there are the following entries saved in this table:

1     NULL   abc     def     NULL
2     1      test    NULL    NULL
3     1      NULL    NULL    xyz

Now I’d like to search in all rows A which haven’t got any rows B which are pointing to them (B.parent_id = A.id). In addition the row values should be either the ones that are present in the current row or if there is a NULL, the values of the parent should be considered.

To illustrate my requirements I’d like to show some examples:

SEARCH(col0=test) => #2 (#1 has some children, #3.col0 = abc (inherited from #1))
SEARCH(col1=def) => #2, #3 (#1 has some children)
SEARCH(col2=xyz) => #3 (#1 has some children, #2.col2 = NULL (inherited from #1))

Does anyone know how to implement such a search in MySQL?

Answer 1

SELECT
# if first table has no value, use parent table
IF(t1.col0, t1.col0, t2.col0) as virtcol0,
IF(t1.col1, t1.col1, t2.col1) as virtcol1,
IF(t1.col2, t1.col2, t2.col2) as virtcol2
FROM table as t1
LEFT JOIN table as t2 ON t1.parent_id = t2.id
LEFT JOIN table as t3 ON t1.id = t3.parent_id
# t3 would be children of t1. We don't want t1 to procreate. :)
WHERE t3.id IS NULL
# Your actual search goes here:
AND virtcol0/1/2 = whatever

Fast? No. Best index use you can get out of this are the joins on id/parent_id.

If you have a lot of data and small result sets, you can query the columns directly on an index and then run checks for parents and children in separate queries. That’d be a lot faster than running the above query on a huge table.