To round down (truncate) you can use the third argument of the ROUND function.

https://docs.microsoft.com/en-us/sql/t-sql/functions/round-transact-sql?view=sql-server-2017#c-using-round-to-truncate

Hi Ghazy, Thanks for posting this question. This is a very common problem with a straightforward solution. I would solve this by doing the following in Discovery Hub.

I have a sales transaction table with a field called Sales Amount. 

In my example, I created two additional fields. The first is named SalesAmount(NoDecimal) where I have edited the field and reduced the fractional digits to zero. 

I added a custom transformation to this field. 

Next, I added another field with the same, zero fractional digits, called SalesAmount(Floor). 

This time I added a transformation but used the FLOOR function. As you can see in the snip below I am using FLOOR() with the field I want to round down in the parenthesis. See here for more info on using FLOOR.

So my fields in Discovery hub appear like this

And Finally, the data appears like this: 

Hope that helps!