Question

I’m trying to make a SELECT on three relational tables like these ones:

table_materials
-> material_id
- material_name

table_props
-> prop_id
- prop_name

table_materials_props
- row_id
-> material_id
-> prop_id
- prop_value

On my page, I’d like to get a result like this one but i have some problem with the query:

material    prop A    prop B    prop C    prop D    prop E
wood        350       NULL      NULL      84        16
iron        NULL      17        NULL      NULL      201
copper      548       285       99        NULL      NULL

so the query should return something like:

material   prop_name     prop_value
wood       prop A        350
wood       prop B        NULL
wood       prop C        NULL
wood       prop D        84
wood       prop E        16
// and go on with others rows

i thought to use something like:

SELECT *
FROM table_materials AS m
INNER JOIN table_materials_props AS mp
ON m.material_id = mp.material_id
INNER JOIN table_materials_props AS p
ON mp.prop_id = p.prop_id
ORDER BY p.prop_name

the problem is the query doesn’t return the NULL values, and I need the same prop order for all the materials regardless of prop values are NULL or not

I hope this example is clear!

Answer 1

Cross-join (or inner join w/o a condition) all the options, then left join those to the associative entity table. The left join will preserve prop/materials even when the combinations don’t exist in the associative table.

Untested:

SELECT * 
FROM table_materials AS m
INNER JOIN table_props as p 
LEFT JOIN table_material_props AS mp 
ON p.prop_id = mp.prop_id 
AND 
m.material_id = mp.material_id;
ORDER BY p.prop_name