How it works...

For each column we wish to aggregate, we create a table variable, __Column1, __Column2, __Column3, and __Column4. Each of these table variables follows the same pattern. SELECTCOLUMNS is used to select all rows in the R01_Table table currently in context for a particular column. The columns chosen are Value1, Value2, Value3, and Value4. Critically, each of these selected columns is essentially renamed to the same column name, __Column, within our SELECTCOLUMNS statements. We then use the UNION function to create another table variable, __Table, which connects each of our other table variables, __Column1, __Column2, __Column3, and __Column4, together. The end result is that the __Table variable now contains a single-column table with all of the values from all of our selected columns. This single column is called __Column. Thus we can now use any of our iterative aggregation functions, such as SUMX, to return an aggregation against our __Table variable for the __Column column.