Home >> FAQs/Tutorials >> Oracle DBA FAQ >> Index

Oracle DBA FAQ - Understanding SQL SELECT Query Statements

By: FYIcenter.com

Part:   1  2  3  4  5   6  7  8 

(Continued from previous part...)

How To Write a Query with an Inner Join?

If you want to query from two tables with an inner join, you can use the INNER JOIN ... ON clause in the FROM clause. The following query returns output with an inner join from two tables: employees and departments. The join condition is that the department ID in the employees table equals to the department ID in the departments table:

SQL> SELECT employees.first_name, employees.last_name, 
  2  departments.department_name 
  3  FROM employees INNER JOIN departments 
  4  ON employees.department_id=departments.department_id;
FIRST_NAME           LAST_NAME            DEPARTMENT_NAME
-------------------- -------------------- ---------------
Steven               King                 Executive
Neena                Kochhar              Executive
Lex                  De Haan              Executive
Alexander            Hunold               IT
Bruce                Ernst                IT
David                Austin               IT
Valli                Pataballa            IT
......

Note that when multiple tables are used in a query, column names need to be prefixed with table names in case the same column name is used in both tables.

How To Define and Use Table Alias Names?

When column names need to be prefixed with table names, you can define table alias name and use them to prefix column names as shown in the following select statement:

SQL> SELECT e.first_name, e.last_name, d.department_name
  FROM employees e INNER JOIN departments d 
  ON e.department_id=d.department_id;
FIRST_NAME           LAST_NAME            DEPARTMENT_NAME
-------------------- -------------------- ---------------
Steven               King                 Executive
Neena                Kochhar              Executive
Lex                  De Haan              Executive
Alexander            Hunold               IT
Bruce                Ernst                IT
David                Austin               IT
Valli                Pataballa            IT
......

How To Write a Query with a Left Outer Join?

If you want to query from two tables with a left outer join, you can use the LEFT OUTER JOIN ... ON clause in the FROM clause. The following query returns output with a left outer join from two tables: departments and employees. The join condition is that the manager ID in the departments table equals to the employee ID in the employees table:

SQL> set NULL 'NULL'
SQL> SELECT d.department_name, e.first_name, e.last_name 
  2  FROM departments d LEFT OUTER JOIN employees e 
  3  ON d.manager_id = e.employee_id;
DEPARTMENT_NAME           FIRST_NAME           LAST_NAME
-------------------- -------------------- --------------
Administration            Jennifer             Whalen
Marketing                 Michael              Hartstein
Purchasing                Den                  Raphaely
Human Resources           Susan                Mavris
Shipping                  Adam                 Fripp
IT                        Alexander            Hunold
......
Treasury                  NULL                 NULL
Corporate Tax             NULL                 NULL
Control And Credit        NULL                 NULL
Shareholder Services      NULL                 NULL
Benefits                  NULL                 NULL
Manufacturing             NULL                 NULL
Construction              NULL                 NULL
......

Note that a left outer join may return extra rows from the first (left) table that do not satisfy the join condition. In those extra rows, columns from the second (right) table will be given null values.

The extra rows returned from the left outer join in this example represents departments that have no manager IDs.

(Continued on next part...)

Part:   1  2  3  4  5   6  7  8