![]() ![]() In the example, we use the concatenation operator to from the full names of the employees by concatenating the first name, space, and last name. Note that the concatenation operator || concatenates multiple strings into a single string. The employees table has two roles: employees and managers.īecause we used the INNER JOIN clause to join the employees table to itself, the result set does not have the row whose manager column contains a NULL value. The statement used the INNER JOIN clause to join the employees to itself. ORDER BY manager Code language: SQL (Structured Query Language) ( sql ) INNER JOIN employees m ON m.employeeid = e.reportsto To get the information on who is the direct report of whom, you use the following statement: SELECT m.firstname || ' ' || m.lastname AS 'Manager',Į.firstname || ' ' || e.lastname AS 'Direct report' FROM employees e In case an employee does not report to anyone, the ReportsTo column is NULL. If an employee reports to a manager, the value of the ReportsTo column of the employee’s row is equal to the value of the EmployeeId column of the manager’s row. The ReportsTo column specifies the reporting relationship between employees. The employees table stores not only employee data but also organizational data. We will use the employees table in the sample database for demonstration. You often use self-join to query parents/child relationship stored in a table or to obtain running totals. Only one table is involved in the self-join. The self-join compares values of the same or different columns in the same table. You use self-join to create a result set that joins the rows with the other rows within the same table.īecause you cannot refer to the same table more than one in a query, you need to use a table alias to assign the table a different name when you use self-join. The self-join is a special kind of joins that allow you to join a table to itself using either LEFT JOIN or INNER JOIN clause. Note that you should be familiar with INNER JOIN and LEFT JOIN clauses before going forward with this tutorial. ![]() The query rewrite mechanism is continually being updated from one release to the next and the details about how it works can be found in the SQL Transaction Processing Manual for Teradata 13.0.Summary: in this tutorial, you will learn about a special type of join called SQLite self-join that allows you to join table to itself. This is simply a technique you can put in your toolkit to when you encounter such a situation. This is typically one of my last resorts when I have a query plan that simply doesn't eliminate extraneous records earlier enough in the plan and results in too much data being scanned and carried around through the various SPOOL files. This is not to say that your standard approach should be to run with this through out your code. The optimizer is then obligated to resolve the GROUP BY aggregate before it can consider resolving the join between the two tables in your example. You can force the optimizer to process a derived table by simply placing a GROUP BY in your derived table. Even if you have a derived table such as the one in your example. In certain situations, based on data demographics, joins, indexing and statistics you may find that the optimizer is not eliminating records in the query plan when you feel that it should. Depending on the availability of statistics and indexes for the tables in question the query rewrite mechanism in the optimizer will may or may not opt to scan Table2 for records where val < 100 before scanning Table1. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |