display only not null value with SSP

ECEGROUPE

Im using datatable with SSP, what i want is to display only the data where mydata is not null. So in my serverside script i add this line before the processing() method (based of this post here )
->where('mydata', null)

It work well to display only the data where mydata is null but i want to do the opposite, only display the data where mydata is not null. I have tried many things like
->where('mydata', not null)
but none of them work, can you help me pls ?