SAN FRANCISCO  In many ways, MySQL embodies the ideals of the populist software movement known as open source, in which a program’s creator releases it to the world free of charge, and legions of volunteers contribute improvements that are also freely shared.

The start-up company came out of nowhere, building a database application beloved by vibrant, young Internet companies. Logging in from homes scattered around the globe, its workers seemed more a part of a virtual commune than a corporate monolith, and they relished taking on proprietary software giants like Microsoft.

But like most open-source companies, MySQL’s sales, tied to support deals, never matched the astronomical number of downloads for its product, about 60,000 a day. In January 2008, the founders decided to sell the company for $1 billion to Sun Microsystems. And this year, Sun agreed to sell itself to Oracle, which makes database software aimed at larger companies and tougher jobs, for $7.4 billion.

Now, disagreement over the value of MySQL  both as a stand-alone entity and as part of a big company  lies at the heart of a bitter public battle between Oracle and the European Union over the Sun acquisition. The fight illuminates a larger truth about open-source companies: their societal and strategic importance far exceeds their financial value as operating businesses.