The database version number is stored in the wp-options table under the option name db_version.
3.9.1 = 27916 3.9 = 27916 3.8 = 26691 3.7 = 25824 3.6 = 24448 3.5 = 22441 3.4.2 = 21707 3.4.1 = 21115 3.4 = 20596 3.3 = 19470 3.2.1 = 18226 3.2 = 18226 3.1.4 = 17516 3.1.3 = 17516 3.1.2 = 17516 3.1.1 = 17516 3.1 = 17056 3.0.5 = 15477 3.0.4 = 15477 3.0.3 = 15477 3.0.2 = 15477 3.0.1 = 15477 3.0 = 15260 2.9.2 = 12329 2.9.1 = 12329 2.9 = 12329 2.8.6 = 11548 2.8.5 = 11548 2.8.4 = 11548 2.8.3 = 11548 2.8.2 = 11548 2.8.1 = 11548 2.8 = 11548 2.7.1 = 9872 2.7 = 9872 2.6.5 = 8204 2.6.3 = 8204 2.6.2 = 8204 2.6.1 = 8204 2.6 = 8201 2.5.1 = 7796 2.5 = 7558 2.3.3 = 6124 2.3.2 = 6124 2.3.1 = 6124 2.3 = 6124 2.2.3 = 5183 2.2.x = 5183 2.2 = 5183 2.1.3 = 4773 2.1.x = 4773 2.1 = 4772 2.0.11 = 3441 2.0.x = 3441 2.0 = 3441 1.5.x = 2541 1.x = 2540