The upgrade from 1.7 is not straight forward. Here is the process you must do:
Restore a backup of your 1.7.9 database
Run the 1.7to1.8.sql upgrade
Run the commentsfix.sql
Run the structurefix.sql
Run the 1.7to1.8.sql again
Run the 1.8to1.9.sql
Do not run the comments and structures a second time as they will result in data corruption.
This upgrade is detailed on the UpgradeTo18 / UpgradeTo19 pages.