SQL JOIN can be used to fetch data from more than one tables based the relationship between columns in tables . Tables are usually related each with primary keys . primary key can be a column with unique value in it .
There are mainly 4 SQL JOINs
JOIN: Return rows when there is at least one match in both tables
LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table
RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table
FULL JOIN: Return rows when there is a match in one of the tables