How to check if an item exists

Hi

I want to use xsl to generate a web page from xml data I retrieve from a DB
with a java program I made.

Some of data to be showed could exists or not then I need to make an if or
something else to check if data exists. If data exists I want to show a
table with soe other data, if not..I want to show a <div> with a "no data"
message.

How could I do ?

Thanks in advance

<jl>