I’ve written a virtual table to view spreadsheet data inside Excel workbook
(.xlsx) files as SQlite tables. I want to implement an SQL function, attr(),
which the virtual table overrides to give access to the raw attributes of the
cell being returned. I’m looking for a robust way to find the actual column
refered to by an overriden function, not just the value returned by the xColumn
call.
Example:
sqlite> .load xlsxvtab.so # creates
xlsxvtab module and attr() function
sqlite> create virtual table summary using
xlsxvtab(‘expenses.xlsx’,’sheet1’,’F20’, ‘H32’);
sqlite> select F,G,H,attr(H,3) from summary; # show formula used
to calculate column H.
The issue is that attr gets called with 2 values and I need to divine that the
first value came from column H of the current row of the cursor opened by this
select statement. The hack I’m using now is to give the values returned by the
xColumn method a subtype equal to the column number. In the attr() function I
retrieve the subtype and re-fetch that column from the current row of the last
cursor open on that table.
Various pitfalls with this technique:
- Subtype numbers are limited to the range 0-255.
- Assumes all functions called before xNext() method called again.
- Complex queries with multiple cursors?
Any suggestions?
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