In this tutorial, you have learned how to use the Oracle LEFT JOIN clause to retrieve data from multiple tables. ![]() Note that for the inner join, the condition placed in the ON has the same effect as it is placed in the WHERE clause. In this case, the query returns all orders but only order 58 had the salesman data associated with it. Now if you move the condition from the WHERE clause to the ON clause of the LEFT JOIN: SELECT The following statement gets the order and salesman data of order 58. For the customers who have not placed any orders, null is used for the columns of the orders table that appear in the SELECT clause, which are order_id, status, and order_date. This statement retrieved all customers and their orders. The following statement demonstrates how to use the LEFT JOIN with the USING statement: SELECT name, The statement that uses the USING clause above is equivalent to the following statement which uses the ON clause: SELECT In this statement, the columns listed in the USING clause must be presented in both T1 and T2 tables. The following shows the syntax of the LEFT JOIN with the USING clause: SELECT The USING clause specifies which column to test for equality when you join tables. The following statement uses LEFT JOIN clauses to join three tables: orders, employees and customers: SELECTĬustomers.customer_id = orders.customer_idĬode language: SQL (Structured Query Language) ( sql ) Oracle LEFT JOIN with USING clause For the rows in the orders table that do not have a matching row in the employees table, NULL values are used. The result includes all rows from the orders table. The following picture illustrates the result: LEFT JOIN employees ON employee_id = salesman_id The following statement retrieves all orders and employee data from both orders and employees tables: SELECT The salesman_id column is nullable, meaning that not all orders have a sales employee who is in charge of the orders. It has the salesman_id column that references to the employee_id column in the employees table. The orders table stores the sales order header data. See the following orders and employees tables in the sample database: ![]() PostgreSQL supports inner join, left join, right join, full outer join, cross. ![]() The common columns are typically the primary key columns of the first table and foreign key columns of the second table. In other words, a left join returns all rows from the left table and matching rows from the right table. PostgreSQL join is used to combine columns from one ( self-join) or more tables based on the values of the common columns between related tables. In case a row in the T1 table does not have any matching rows in the T2 table, the query combines column values from the row in the T1 table with a NULL value for each column in the right table that appears in the SELECT clause. If a pair of rows from both T1 and T2 tables satisfy the join predicate, the query combines column values from rows in both tables and includes this row in the result set. The query compares each row in the T1 table with rows in the T2 table. In this query, T1 is the left table and T2 is the right table. The following statement illustrates the syntax of the LEFT JOIN clause when joining two tables T1 and T2: SELECTĬode language: SQL (Structured Query Language) ( sql ) Summary: in this tutorial, you will learn how to use the Oracle LEFT JOIN clause to query data from multiple tables.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |