Question

I wonder if anyone could assist? Table_A below shows an example product in my MySQL product database. I’m having problem working out the query logic to achieve Table_B below. (It’s not the INSERT command I’m having a problem with.)

Table_A

 entry_id | SKU  | Product          | Parent_SKU  | IsVariant
----------------------------------------------------------------
     1    | ABC  | Green T-Shirt    |             | No
     2    | ABCS | Green T-Shirt S  | ABC         | Yes
     3    | ABCM | Green T-Shirt M  | ABC         | Yes
     4    | ABCL | Green T-Shirt L  | ABC         | Yes

Table_B

 child entry_id | parent entry_id
----------------------------------
        2       |       1
        3       |       1
        4       |       1

So far, here’s the query I’ve been running.

INSERT INTO Table_B
(parent_entry_id, child_entry_id)

    SELECT
        Table_A.entry_id,
        (SELECT Table_A.entry_id FROM Table_A
         WHERE Table_A.SKU = Table_A.Parent_SKU
         AND Table_A.IsVariant = 'No')
    FROM Table_A
    WHERE IsVariant = 'Yes'

I know full well the logic is wrong, but after a day of trying to think it through my brain is frazzled. I’ve searched on here and other places as best as I can think, but without any thoughts or answers to help. So can anyone here assist? Should I be thinking the query the another way round?

Answer 1

If you have only need a single level of parent-child relationships (no grandchildren, grandgrand.., etc)
then you can simply do a self join:

SELECT T.entry_id as child_id, T2.entry_id as parent_id

FROM TableA AS T LEFT JOIN TableA AS T2 

ON T.parent_SKU=T2.SKU

If you want to go trough the whole parent-child tree, Oracle and MSSQL have mechanisms that enable you to do that. I’m not familiar with such an option for MySQL.

There more info here:
http://ftp.ntu.edu.tw/MySQL/tech-resources/articles/hierarchical-data.html
http://stackoverflow.com/questions/53108/is-it-possible-to-make-a-recursive-sql-query

Edit:
Ok, some data sanitation first: I do not know if this is an artefact of my import process or if it’s the same at your end, but the parent_sku fields had a newline tacked on. Naturally this caused a failure trying to match a child’s ABC parent_sku to a parents ABC\n SKU. Verify this with select * from TableA where parent_sku like '%\n' . If any columns match, run
SET SQL_SAFE_UPDATES=0;

UPDATE TableA SET parent_sku = REPLACE(REPLACE(parent_sku, ‘\r’, ”), ‘\n’, ”) ;
This will remove the newlines.
Same goes for empty string parent_sku’s – if they aren’t null at your end, make them null.
Here is the revised query – I’ve added some pre-join filtering and it’s faster now. Just double check your \n’s and NULLs and it should produce the correct output:

SELECT child.entry_id as child_id, parent.entry_id as parent_id

FROM 

    (

        select * from TableA 

                -- only children have parent_sku
        where parent_SKU is not null

               -- some extra safety for empty strings, 
                -- can be removed after empties are replaced by NULLs
        and length(parent_SKU) > 0 and length(sku)>0 and length(parent_sku)>0

    )

        AS child 

LEFT JOIN 

    (

        select * from tableA 
                -- only parents don't have parent_sku
        where (parent_SKU is null or length(parent_SKU)=0) 

        and length(sku)>0

    ) AS parent



ON child.parent_SKU = parent.SKU

results