Merge Two Columns

I need to merge two columns with an "if else" formula.

Column A always has a value but Column B has a value sometimes. I need Column C to produce Column B value unless its null then produce column A value if null. Column B is from table 2 and Column A would be from a Query1.

My new column is "QuantityOrdered"

QuantityOrdered: if(([Table2_ColumnB]=null), then([Query1_ColumnA]), else([Table2_ColumnB])))

I think I just made a mess of the formula.

I thought the description was easier than try to post copies of the database. So in the example I need the value in Column O (sheet 3) to be used unless it is null then use Column R on sheet 2 in a new column on the query result I run from sheet 3.

Thanks, that helped!