How to write a check constraint to disallow empty varchar fields? On SQL Server not nullable varchar columns can hold empty strings.

Under ORACLE they can't, because varchar2 treat '' as equivalent to NULL.

If you design a database schema suitable for both RDBMS it seems to be a good idea to add to each not nullable varchar column on SQL Server a constraint which disallows empty strings.

But which is the best way to formulate such a constraint?

I started with 

if OBJECT_ID('varchar_without_empty_cols') &gt; 0 drop table varchar_without_empty_cols
go

create table varchar_without_empty_cols (
id int ,
val varchar(10) not null CHECK (val &lt;&gt; '') 
)
go


insert into varchar_without_empty_cols values (1, ' ')  -- this ought be OK
go
insert into varchar_without_empty_cols values (2, '')   -- this has to violate the check
go
insert into varchar_without_empty_cols values (3, null) -- this violates the not null
go 

select * from varchar_without_empty_cols


But this constraint not only inhibits empty strings, which is intended, but it also inhibits strings consisting of a single character, and that is not what I intend.
To allow strings containing only spaces but disallow empty strings you can use

CREATE TABLE varchar_without_empty_cols
  (
     id  INT,
     val VARCHAR(10) NOT NULL CHECK (DATALENGTH(val) &gt; 0)
  )