Oracle SQL Developer

Retrieving Data With QueriesSee Also:■Oracle Database SQL Reference for detailed information on usingORDER BY with SELECT.6.2.6 Displaying Data From Multiple TablesYou can use SELECT to display data from the multiple tables. This process is referredto as joining the tables. In a join, multiple tables share a similar column.When you retrieve data from multiple tables, you can explicitly identify which table acolumn belongs to. This is important when tables contain columns with the samename. You can use the complete table name to explicitly identify a column, such asemployees.employee_id, or a table alias. Note the use of the table aliases (d, e, andl) to explicitly identify the columns by table in the SQL statement. The alias is definedin the FROM clause of the SQL statement. A table alias is used to simply and reduce thesize of the SQL code.Example 6–6 is an example of querying data from joined tables using ANSI syntax.The first SELECT joins two tables and the second SELECT joins three tables.Example 6–6Selecting Data From Multiple Tables With the ANSI Join Syntax-- the following SELECT statements retrieve data from two tables-- that have a corresponding column (department_id)-- this join uses ANSI syntax, note the use of JOIN and ONSELECT e.employee_id, e.last_name, e.first_name, e.department_id,d.department_name FROM employees eJOIN departments d ON e.department_id = d.department_id;-- the following SELECT retrieves data from three tables-- two tables have the corresponding column department_id and-- two tables have the corresponding column location_idSELECT e.employee_id, e.last_name, e.first_name, e.department_id,d.department_name, d.location_id, l.country_id FROM employees eJOIN departments d ON e.department_id = d.department_idJOIN locations l ON d.location_id = l.location_id;In Example 6–7 the joins use the Oracle-proprietary syntax. There is no performancedifference between the ANSI and Oracle syntax.Example 6–7 Using SELECT to Display Data From Multiple Tables-- the following SELECT statements retrieve data from two tables-- that have a corresponding column (department_id)-- note that the employees table has been aliased to e and departments to dSELECT e.employee_id, e.last_name, e.first_name, e.department_id,d.department_name FROM employees e, departments dWHERE e.department_id = d.department_id;-- the following SELECT retrieves data from three tables-- two tables have the corresponding column department_id and-- two tables have the corresponding column location_idSELECT e.employee_id, e.department_id, d.department_name, d.location_id,l.country_id FROM employees e, departments d, locations lWHERE e.department_id = d.department_id AND d.location_id = l.location_id;6-6 SQL Developer Online Help