Sheet1 of my workbook has some data in columns A and D that I would like
to combine into a single text string, but only if data in Column C is <
"". The string should be made up in the following way: Data in column
A + Underscore sign + Data in column D.

As an example:
* Data in column A: abcdef
* Data in column D: 123456
* End result should be: abcdef_123456

The end result should be listed in Column A of Sheet2. And so, I have
created the following formula and copied it down Column A of Sheet2:

= if(Sheet1!c2<"",Sheet1!a2&"_"&Sheet1!d2,"")

The above formula works fine, except that it creates blank rows in
Sheet2 whenever a record in Sheet1 has data in Column C that is equal to "".

Can anyone suggest a formula that skips all those records in Sheet1
where data in Column C is equal to ""?

Thanks for your help.
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tb