Level: Easy
Vulnerability: SQL Injection
Link: Challenge 1
Level: Easy
Vulnerability: SQL Injection
Link: Challenge 2
Level: Easy
Vulnerability: SQL Injection
Link: Challenge 3
Level: Easy
Vulnerability: SQL Injection
Link: Challenge 4
Level: Easy-Medium
Vulnerability: SQL Injection
Link: Challenge 5
Level: Easy-Medium
Vulnerability: SQL Injection
Link: Challenge 6
Level: Medium
Vulnerability: SQL Injection - Error based
Link: Challenge 7
Level: Medium
Vulnerability: SQL Injection - Time based
Link: Challenge 8
Level: Medium-Hard
Vulnerability: SQL Injection - Blind
Tip: A python script could help you a lot in here.
Link: Challenge 9
Level: Hard
Vulnerability: SQL Injection - Blind
Link: Challenge 10
Level: Hard
Vulnerability: SQL Injection - Filter bypass
Link: Challenge 11
There are many ways to exploit SQLI, playing different CTF you could find those ways and learn from them.