How can I show the column names of a table as options in a dropdown in appsheet?

[Calvin_Lopez]

This is the context:

In appsheet I have a table called 'Inventory' which contains 49 columns, which are the following:

- Name: is the key column.
- The remaining 48 columns are divided into two groups of 24 columns that share the same names but in the first half have a letter 'L' and the other half a letter 'B', which are:

*T17
*T17.5
*T18
*T18.5
*T19
*T19.5
*T20
*T20.5
*T21
*T21.5
*T22
*T22.5
*T23
*T23.5
*T24
*T24.5
*T25
*T25.5
*T26
*T26.5
*T27
*T27.5
*T28
*T29

-------------------------

I am creating a view of a table called 'Sales' whose columns are as follows:

- Name: it is a reference of the 'Inventory' table.
- Origin: Saves a string with only two possible options which are 'Local' and 'Warehouse' that tells us to which group of the 'Inventory' table it belongs to (whether 'L' or 'B' since they are the first letter of each option).
- Size: It must show a drop-down list of all the column names of the 'Inventory' table whose value is greater than zero and differs from the option chosen in the 'Origin' column.

But I don't know how to make the 'Size' column, any ideas?

[James_Burns]

I don't believe it's possible to achieve what you describe (see the responses to this question) other than to manually include each one in the expression you use. 
I would suggest structuring your data differently, using a unique id field as the key and then have a name and size field (and origin possibly too if you prefer not to determine the origin by expression only). The expression to show only certain sizes should be pretty straightforward from there.