A join is a query that combines data from two or more tables in a relational database. Joins are used to retrieve data from multiple tables with a single query, allowing the user to query data in a more efficient way. A join is specified by a join condition, which is typically a comparison between the values of two columns in the same or different tables. The columns being compared must share the same data type. The most common type of join is an inner join, which returns only matching rows from both tables. Outer joins can be used to return all rows from one table, regardless of whether or not a match exists in the other table. Inner Joins: Inner joins are used to return only rows that match in both tables. An inner join requires a join condition to be specified, which is used to determine which rows from the two tables should be returned. Outer Joins: Outer joins are used to return all rows from one table, regardless of whether or not the rows match in the other table. Outer joins are usually used when you need to return all rows, even if there are no matches in the other table. Cross Joins: Cross joins are used to return all possible combinations of rows from the two tables. Cross joins do not require a join condition, as they will return all combinations of the two tables. Self Joins: Self joins are used to join a table to itself. Self joins are used to compare values in a column to other values in the same column. Self joins require a join condition to be specified in order to determine which rows should be returned.

JOIN is a clause in SQL used to combine rows from two or more tables, based on a related column between them. The JOIN clause allows us to retrieve data from two or more tables, based on a related column in those tables. It is a part of the SELECT statement used to select data from one or more tables. In general, there are four types of JOINs – INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL OUTER JOIN. They are different from each other in terms of what type of records they retrieve from the two tables. An INNER JOIN will return only those records from the two tables that match a given criteria. For example, if we are joining two tables on the basis of student_id, an INNER JOIN will return the records of those students who have a record in both tables. A LEFT JOIN will return all the records from the left table, even if there is no matching record in the right table. It is useful when we want to retrieve all the records from one of the tables, even if there is no matching in the other table. A RIGHT JOIN is the opposite of a LEFT JOIN, it will return all the records from the right table, even if there is no matching record in the left table. A FULL OUTER JOIN will return all the records from both the tables, even if there is no matching record in either of the tables. It is useful when we want to retrieve all the records from both the tables, even if there is no matching in either table. In conclusion, the JOIN clause is used to retrieve data from two or more tables based on a related column between them. It is generally used in SELECT statements to retrieve data from multiple tables.