Add 'CHECK( length(fieldname) > 0 )' to a field

Hello,

I have to let postgreSQL check the length of some fields in a database.
So I want to add the SQL Code "CHECK( length(fieldname) > 0 )" to some
fields so that the check will be executed during INSERT statement.
With the properties dialog I can't add the CHECK statement, so maybe it
isn't possible or I have to take a look to something. But to what?

Thank you in advance

Daniel Seichter