Google Sheets Query to unpivot and fill null values

Question

I am writing a complex Google Sheets formula, which I think I can simplify using the Query function. The Query formula takes two inputs: 1) data, 2) query. The data consists of about 20 columns where the odd-numbered columns are categories and even-numbered columns are the values of that category. (Note that there are a fixed number of rows, but various columns have variable number of non-blank entries.)

Column A	Column B	Column C	Column D
1st Category	Value B1	2nd Category	Value D1
	Value B2		Value D2
	Value B3

My dream output is as follows:

Categories	Values
1st Category	Value B1
1st Category	Value B2
1st Category	Value B3
2nd Category	Value D1
2nd Category	Value D2
...etc.	...etc.

Thanks for any thoughts !

Note: Common Table Expressions can't seem to be used in the Query function.

Add'l note: I don't mind and partially expect that I will have to repeat the query for the transformation of Column A and Column B ten times to get all the data, which is fine. The simpler first-step question is how to do that.

Sample Google Sheets for reference/work: https://docs.google.com/spreadsheets/d/132E5CYwcv-ovWbZTnYqRMMqrR2X2pDv28Pj2bdW7w1k/edit?usp=sharing

Answer 1

use:

=ARRAYFORMULA(QUERY(SPLIT(FLATTEN(IF(
 TRANSPOSE(QUERY(TRANSPOSE(Data!B1:L5), "skipping 2", ))="",,
 TRANSPOSE(QUERY(TRANSPOSE(Data!A1:L1), "skipping 2", ))&"×"&
 TRANSPOSE(QUERY(TRANSPOSE(Data!B1:L5), "skipping 2", )))), "×"), 
 "where Col2 is not null order by Col1", ))