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This isn't meant to be a comprehensive discussion of all databases. Instead, its a comparison of certain features available in the free offerings from the dominant players in the relational market.
|Oracle XE||MS SQL Express||IBM DB2 Express-C|
|Vendor-provided patches & updates||No||Yearly||Fixpacks|
|Free for production environments||Yes||Yes||Yes||Yes|
|Max data size||4gb||4gb||Unlimited|
|Max RAM usable||1gb||1gb||2gb||4gb|
|Max # CPUs||1||1||2||4|
|Max # concurrent connections||none (default=20)||5||no limit||no limit|
|Vendor 24/7 support||No||No||No||Yes|
|Non-disruptive DB upgrade||No||Yes||Yes||n/a|
|DB Upgrade price||$5800/cpu/yr||$3500/svr/yr||$3k/svr/year|
|DB Upgrade complexity||Medium||Medium||near-zero|
|Native XML storage & processing||manual||Robust||Robust|
The reason I used two columns for DB2 is because it has an interesting license model. The advanced features exist in the Express-C software, but they are disabled by default. Purchasing and installing a licence key activates them automatically–no install required. Interesting option if you are cash-strapped during development and the early phases of a project, but want to add these features down the road with minimal work.
Notice that DB2 allows more RAM, disk, and CPU to be used by the database than its competition. This is a good signal that IBM recognizes its underdog status in the market and is trying to do something about it. Good for the consumer!