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 |
                  ...