LimeSurvey 1.8 database layout documentation
From LimeSurvey Manual
The following Excel sheet contains a rough documentation of the old LimeSurvey 1.8 database layout (for the newer Limesurvey 2.x version see LimeSurvey_2.5_database_layout_documentation). The headlines are German but you should have no problems in understanding their meaning. The file is attached to this wiki page (see bottom of this page to download the file).
One of our users also supplied us with an ER diagram. You can download the full size image here: