As @Adnan says, the worst is "Full compromise and the existence of a backdoor.". Why? While playing with SQL alon can't lead to any compromise of the system itself (just reload the datatabase from a backup and you ought to be set), the programs using SQL and interacting with the system have much higher capabilities.
For example, there's a chance that you have a table containing PHP template snippets. An attacker can edit that and add malicious code. If anything from the SQL tables is being executed (or even dumped to a file -- if the file can be dumped to the web folder, then an attacker could use it to inject and run his own PHP), then you may have a problem.
So find out where all the data in the tables is used. If, at any point, it is being saved or executed, see if this is exploitable (in the case of the former, it depends if the attacker can put arbitrary text in the web folder using it. In the case of the latter, it's almost always a problem). If so, there's a chance the system was compromised, and you may want to do a clean install.
If not, then just look for problems in the tables and fix them if present. Make sure you change your username/password (and check for rogue accounts)
An interesting thing to note is that in SQLite, the
ATTACH DATABASE command can be used to directly introduce vulnerabilities.
SPOOL command in SQL* Plus can be used if the injection allows for multiple commands to be executed.
If you're using either of the above two, a clean install would be good regardless of how the data is being used.