Extending Values in Joined Tables with MYSQL - mysql

Question

I currently have a table of concentrations, which are linked to a table of elements. There is a concentration table
|conc_id|element_id|conc|
and and element table
|element_id|symbol|
The concentration table only has element_ids for each conc_id that have non-zero concentrations.
What I am trying to do is create a query which will, for each concentration_id, list all the elements in order, with their concentration, regardless of whether they're non-zero or not. I have tried this in a number of increasingly complicated ways (starting with a RIGHT JOIN) but this always outputs a NULL conc_id when there is none of that element in it. The output I am looking for is something like this:
|conc_id|element_id|symbol|conc|
|1 |1 |H |1.2 |
|1 |2 |He |NULL|
|1 |3 |Li |2.3 |
...
|3 |1 |H |4.5 |
|3 |2 |He |NULL|
|3 |3 |Li |NULL|
...
And so on. Is there a way I can do this without having NULL conc_id?
Thanks for any help in advance...

Answer 1

First, you have to build a table that contains all combinations between conc_id and element_id:
SELECT DISTINCT conc_id, elements.element_id FROM concentrations, elements;
Which gives you the following table:
| conc_id | element_id |
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
...
| 3 | 1 |
| 3 | 2 |
| 3 | 3 |
...
Once you have such a table, joining it with concentrations and elements should be fairly simple. For example:
SELECT combinations.conc_id, combinations.element_id, symbol, conc
FROM
(SELECT DISTINCT conc_id, elements.element_id FROM concentrations, elements) AS combinations
LEFT JOIN concentrations ON (combinations.conc_id = concentrations.conc_id AND combinations.element_id = concentrations.element_id)
LEFT JOIN elements ON (combinations.element_id = elements.element_id);
Result:
| conc_id | element_id | symbol | conc |
| 1 | 1 | H | 1.2 |
| 1 | 2 | He | NULL |
| 1 | 3 | Li | 2.3 |
...
| 3 | 1 | H | 4.5 |
| 3 | 2 | He | NULL |
| 3 | 3 | Li | NULL |
...

Answer 2

To be honest, I think the best way would be to actually fill in the missing data with zero values in the conc column. Otherwise, you're trying to hack it to display data that you don't actually have and it would be much more tangible than keeping null values anyway...
Assuming there are no performance consideration that this would impede, of course.

Answer 3

Assuming you do not have a simple list of all the conc_id somewhere, you could try something like:
SELECT c.conc_id, e.element_id, e.symbol, c2.conc
FROM
(SELECT DISTINCT conc_id FROM concentration c) c
INNER JOIN element e
LEFT JOIN concentration c2 ON c2.conc_id = c.conc_id AND c2.element_id = e.element_id
ORDER BY c.conc_id, e.element_id
The logic is:
Get a list of all unique conc_id derived table c
Do a cross join against element (for each row of c, list all row of element)
Left join against the full concentration table.
If you have another table with all the conc_id, you could avoid the derived table entirely.

Answer 4

select conc_id, conc.element_id, symbol, concentration from concentration_table as conc
join element_table as elem on conc.element_id = elem.element_id
order by conc.conc_id asc
This gives me this table:
conc_id element_id concentration element_id symbol
1 1 1.2 1 H
1 2 0 2 He
1 3 2.3 3 Li
3 1 4.5 1 H
3 2 0 2 He
3 3 0 3 Li
And these are the starting tables:
element_table:
element_id symbol
1 H
2 He
3 Li
concentration table:
conc_id element_id concentration
1 1 1.2
1 2 0
1 3 2.3
3 1 4.5
3 2 0
3 3 0
EDIT: edited the query to obtain correct table values requested