Over Months now I have tried a lot of different Design-Tools for Databases and always remarked that they don't fit to my Requirements due to simple but unnecessary Restrictions in building Connections between Tables.

A very basic Fault of some Tools is that Connections can be build only between Tables without declaring the concerned Fields, so the whole Design is unclear and neither usable for graphical Demonstration nor for Programming or for automatically Creation of Tables, Scripts, UML- or SQL-Files.

Another Fault is that Connections mostly are defined as 1:1, 1:n or m:m.
m:m Connections surely require an extra-Table but the Existence and Usage of a mm-Table doesn't always mean that mm-Connections are used without Restrictions. This Point is omitted in many Tools and in EVERY TOOL I tried there is no Possibility to enter the right Connections between Tables in all required Kinds.

PMA-Designer is a Tool inside the web-based DBMS  PhpMyAdmin.
It allows graphical Designing of Connections between the Tables of a Database.
This means that the Tables of the Database are automatically read and displayed which often is available in UML-Tools only as payed Addon of free versions. Nevertheless PhpMyAdmin isn't able to create UML, XMI or something like that. Shemes can be exported as PDF but the graphical Output is poor because Tables aren't arranged correct (at least my tries did show wrong results). Unnecessary Tables for the visual Design can be hidden, but these settings concern only the Designer and are ignored in any Export.

For simple Visualization of Tables and Connections the PMA-Designer is very fine, it allows fast Working especially because Tables are displayed and updated automatically. One nice Feature is that direct Connections between Tables are also linked in SELECTs, so a list of rows allows verifying of connected Values just by clicking on the Field - so you're redirected to the connected Row of the foreign Table.

Created Designs in PMA are stored in an own Database inside MySQL and don't need much Space. Regrettable the amount of Export-Formats is as poor as the Quality of  exported PDFs but a Screenshot of the designed relations is good for Documentation because the visual style inside the tool is very good.

Features PMA-Designer keeps missing:
The possibility to insert the type of the connections. In some other tools at each end of an arrow labels can be given if it's a 1:1, 1:n or a m:m connection.

Building UML-Files with PMA-Designer isn't possible yet. Also any Kind of Graphics can't be exported. Concerning import it's the same I suppose.

UML is used in many Tools but I never found one that is able to read the Files from other Tools, so UML has to many Possibilities in Notation or Developers until now didn't tried enough to interpret foreign or possible Notations visually. Due to the Fact that often a big Bunch of Functions is referenced in UML the Code isn't really translatable at all  - I'm missing Divisions for Structure, visual Style and the functional Refernces, in most cases UML is only used as ineffective Method to store application-dependent Data, PMA-Designer stores those Data in a Database without the (often very huge) XML-Overhead.