OK, it is possible to do in vim, but I wouldn't say it's the best tool for the job - the best tool would be sed
In vim, make sure you're in command mode (press ESC a couple of times), and then type:
:%s/xyz_/abc_/g
the colon is for a command, the % is for a range (means the entire file - you could do this over specific lines if you wanted). Everything else is like a sed regex.
To do it with sed on the command line, you could type this:
sed -ie "s/xyz_/abc_/g" sqlfile
you could expand the regular expression to be more specific if xyz_ appears in places you don't want changed.