displaying data in dropdownlist using sql with querystring - sql

Question

I have three tables:
tbprd -- prdcod int PK, prdtit v(100), prddefuom int FK to prduomcod (tbprduom)
tbuom -- uomcod int PK, uomnam v(100)
tbprduom -- prduomcod int PK, prduomprdcod int FK prdcod(tbprd), prduomuomcod int FK uomcod(tbuom)
What I need is to display uomnam in dropdownlist display field and uomcod in data value field from prdcod which I have passed through query string but if the chosen prdcod is 4 then the dropdownlist should display all the uomnam matching w.r.t all matching prdtit
Query I am using:
select uomcod, uomnam
from tbuom, tbprd, tbprduom
where prdcod = 4
and prduomuomcod = uomcod
and prddefprduomcod = prduomcod
Look at the images for more clearer idea about what I want and table data
https://drive.google.com/drive/folders/17p3d1WXMppndIq1bXf6EwShLWBrh-IsC?usp=sharing

Answer 1

Based on what I understand, you want to get all products that have the same title as the product with the given prdcod. If that's correct, then here's one way to do it:
Write a query to get the prdtit value for prdcod = 4 (return one column from one row):
SELECT prdtit
FROM ...
Use this value to filter the rest of the rows in the tbprd table:
SELECT *
FROM tbrpd
WHERE prdtit = (<Query in Step 1>)
Add two joins to the tbprduom and tbuom tables to Step #2 to get the remaining info:
SELECT tbuom.uomcod, tbuom.uomnam
FROM tbrpd
WHERE prdtit = (<Query in Step 1>)
LEFT JOIN tbprduom ON ... <PK/FK fields) -- Get uomcod for products
LEFT JOIN tbuom ON ... <PK/FK fields) -- Get "uom" fields to return
Give it a try and see how far you get. If you get stuck on a step, just let me know.