A Join is used to compare and combine, this means literally join and return specific rows of data from two or more tables in a database.
There are three types of joins in SQL that are used to write the subqueries.
Self Join : This is a join in which a table is joined with itself, especially when the table has a foreign key which references its own primary key.
Outer Join : An outer join helps to find and returns matching data and some dissimilar data from tables.
Equi-join : An equijoin is a join with a join condition containing an equality operator. An equijoin returns only the rows that have equivalent values for the specified columns.
