Methods and implementation of join operation
The join operation is used in relational databases to combine rows from two or more tables based on a related column between them. There are several methods and implementations of the join operation, including:
1. Inner Join: This is the most common type of join operation. It returns only the rows from the tables that have matching values in the specified column(s). The implementation involves comparing the values of the specified column(s) for each pair of rows from the tables and including the rows in the result if they have the same value.
2. Left Join: This type of join returns all rows from the left table and the matching rows from the right table. If there is no match, NULL values are returned for the columns of the right table. The implementation is similar to the inner join, but all the rows from the left table are included in the result, regardless of whether they have matching values.
3. Right Join: This type of join is the reverse of the left join. It returns all rows from the right table and the matching rows from the left table. If there is no match, NULL values are returned for the columns of the left table. The implementation is similar to the left join, but all the rows from the right table are included in the result, regardless of whether they have matching values.
4. Full Outer Join: This type of join returns all rows from both tables, including the non-matching rows. If there is no match, NULL values are returned for the columns of the non-matching table. The implementation involves combining the results of the left join and right join.
5. Cross Join: This type of join returns the Cartesian product of the two tables, i.e., all possible combinations of rows from the tables. The implementation does not require a specific join condition and returns all combinations of rows.
The implementation of join operation depends on the database management system (DBMS) being used. Most DBMSs provide SQL (Structured Query Language) to write join queries. The SQL syntax for join operation includes the JOIN keyword, followed by the table names and the join conditions using the ON keyword or the WHERE clause. The specific implementation details of join operations vary among different DBMSs, but they generally optimize the join queries to improve performance by using indexes and other techniques.