S.Lott inserted a comment, but it should be an answer: see the same question.
EDIT
Since it wasn't as obvious as I intended it to be, here follows a verbatim copy of S.Lott's answer in the other question:
I'm a big fan of ARGO UML from Tigris.org. Draws nice pictures
using standard UML notation. It does some code generation, but mostly
Java classes, which isn't SQL DDL, so that may not be close enough to
what you want to do.
You can look at the Data Modelling Tools list and see if anything
there is better than Argo UML. Many of the items on this list are
free or cheap.
Also, if you're using Eclipse or NetBeans, there are many
design plug-ins, some of which may have the features you're looking
for.