The built-in Teradata SQL engine in General SQL Parser provides in-depth analysis of an organization's Teradata SQL script(support SELECT/INSERT/DELETE/UPDATE/CREATE.Purpose. Use a SELECT statement or subquery to retrieve data from one or more tables, object tables, views, object views, or materialized views. If part or all of the. What is a self join? Explain it with an example and tutorial. Let’s illustrate the need for a self join with an example. Suppose we have the following table. TECHNOLOGY: Ask Tom. On Top-n and Pagination Queries By Tom Kyte. Our technologist gets more results using ROW_NUMBER, RANK, and DENSE_RANK. SELECTPurpose. Use a SELECT statement or subquery to retrieve data from one or more tables, object tables, views, object views, or materialized views. If part or all of the result of a SELECT statement is equivalent to an existing materialized view, then Oracle Database may use the materialized view in place of one or more tables specified in the SELECT statement. This substitution is called query rewrite. ![]() It takes place only if cost optimization is enabled and the QUERY_REWRITE_ENABLED parameter is set to TRUE. To determine whether query rewrite has occurred, use the EXPLAINPLAN statement. Prerequisites. For you to select data from a table or materialized view, the table or materialized view must be in your own schema or you must have the READ or SELECT privilege on the table or materialized view. For you to select rows from the base tables of a view: The view must be in your own schema or you must have the READ or SELECT privilege on the view, and. ![]() ![]() Whoever owns the schema containing the view must have the READ or SELECT privilege on the base tables. The READANYTABLE or SELECTANYTABLE system privilege also allows you to select data from any table or any materialized view or the base table of any view. To specify the FORUPDATE clause, the preceding prerequisites apply with the following exception: The READ and READANYTABLE privileges, where mentioned, do not allow you to specify the FORUPDATE clause. To issue an Oracle Flashback Query using the flashback_query_clause, you must have the READ or SELECT privilege on the objects in the select list. In addition, either you must have FLASHBACK object privilege on the objects in the select list, or you must have FLASHBACKANYTABLE system privilege. Syntaxselect: :=Description of the illustration ''select. Description of the illustration ''subquery. Description of the illustration ''query_block. Description of the illustration ''with_clause. Note. You cannot specify only the WITH keyword. You must specify at least one of the clauses plsql_declarations or subquery_factoring_clause. Description of the illustration ''plsql_declarations. Description of the illustration ''subquery_factoring_clause. Description of the illustration ''search_clause. Description of the illustration ''cycle_clause. Description of the illustration ''select_list. Description of the illustration ''table_reference. Description of the illustration ''flashback_query_clause. Description of the illustration ''query_table_expression. Description of the illustration ''pivot_clause. Description of the illustration ''pivot_for_clause. Description of the illustration ''pivot_in_clause. Description of the illustration ''unpivot_clause. Description of the illustration ''unpivot_in_clause. Description of the illustration ''sample_clause. Description of the illustration ''partition_extension_clause. Description of the illustration ''subquery_restriction_clause. Description of the illustration ''table_collection_expression. Description of the illustration ''containers_clause. Description of the illustration ''join_clause. Description of the illustration ''inner_cross_join_clause. Description of the illustration ''outer_join_clause. Description of the illustration ''query_partition_clause. Description of the illustration ''outer_join_type. Description of the illustration ''cross_outer_apply_clause. Description of the illustration ''where_clause. Description of the illustration ''hierarchical_query_clause. Chapter 6, "Conditions")group_by_clause : :=Description of the illustration ''group_by_clause. Description of the illustration ''rollup_cube_clause. Description of the illustration ''grouping_sets_clause. Description of the illustration ''grouping_expression_list. Description of the illustration ''expression_list. Description of the illustration ''model_clause. Description of the illustration ''cell_reference_options. Description of the illustration ''return_rows_clause. Description of the illustration ''reference_model. Description of the illustration ''main_model. Description of the illustration ''model_column_clauses. Description of the illustration ''model_rules_clause. Description of the illustration ''model_iterate_clause. Description of the illustration ''cell_assignment. Description of the illustration ''single_column_for_loop. Description of the illustration ''multi_column_for_loop. Description of the illustration ''order_by_clause. Description of the illustration ''row_limiting_clause. Description of the illustration ''for_update_clause. Description of the illustration ''row_pattern_clause. Description of the illustration ''row_pattern_partition_by. Description of the illustration ''row_pattern_order_by. Description of the illustration ''row_pattern_measures. Description of the illustration ''row_pattern_measure_column. Description of the illustration ''row_pattern_rows_per_match. Description of the illustration ''row_pattern_skip_to. Description of the illustration ''row_pattern. Description of the illustration ''row_pattern_term. Description of the illustration ''row_pattern_factor. Description of the illustration ''row_pattern_primary. Description of the illustration ''row_pattern_permute. Description of the illustration ''row_pattern_quantifier. Description of the illustration ''row_pattern_subset_clause. Description of the illustration ''row_pattern_subset_item. Description of the illustration ''row_pattern_definition_list. Description of the illustration ''row_pattern_definition. Description of the illustration ''row_pattern_rec_func. Description of the illustration ''row_pattern_classifier_func. Description of the illustration ''row_pattern_match_num_func. Description of the illustration ''row_pattern_navigation_func. Description of the illustration ''row_pattern_nav_logical. Description of the illustration ''row_pattern_nav_physical. Description of the illustration ''row_pattern_nav_compound. Description of the illustration ''row_pattern_aggregate_func. Semanticswith_clause. Use the with_clause to define the following: PL/SQL procedures and functions (using the plsql_declarations clause)Subquery blocks (using the subquery_factoring_clause)plsql_declarations. The plsql_declarations clause lets you declare and define PL/SQL functions and procedures. You can then reference the PL/SQL functions in the query in which you specify this clause, as well as its subqueries, if any. For the purposes of name resolution, these function names have precedence over schema- level stored functions. If the query in which you specify this clause is not a top- level SELECT statement, then the following rules apply to the top- level SQL statement that contains the query: If the top- level statement is a SELECT statement, then it must have either a WITHplsql_declarations clause or the WITH_PLSQL hint. If the top- level statement is a DELETE, MERGE, INSERT, or UPDATE statement, then it must have the WITH_PLSQL hint. The WITH_PLSQL hint only enables you to specify the WITHplsql_declarations clause within the statement. It is not an optimizer hint. The subquery_factoring_clause lets you assign a name (query_name) to a subquery block. You can then reference the subquery block multiple places in the query by specifying query_name. Oracle Database optimizes the query by treating the query_name as either an inline view or as a temporary table. The query_name is subject to the same naming conventions and restrictions as database schema objects. Refer to "Database Object Naming Rules" for information on database object names. The column aliases following the query_name and the set operators separating multiple subqueries in the AS clause are valid and required for recursive subquery factoring. The search_clause and cycle_clause are valid only for recursive subquery factoring but are not required. See "Recursive Subquery Factoring". You can specify this clause in any top- level SELECT statement and in most types of subqueries. The query name is visible to the main query and to all subsequent subqueries. For recursive subquery factoring, the query name is even visible to the subquery that defines the query name itself. Recursive Subquery Factoring If a subquery_factoring_clause refers to its own query_name in the subquery that defines it, then the subquery_factoring_clause is said to be recursive. A recursive subquery_factoring_clause must contain two query blocks: the first is the anchor member and the second is the recursive member. The anchor member must appear before the recursive member, and it cannot reference query_name. SQL/Databases: What is a self join? What is a self join? Let’s illustrate the need for a self join with an example. Suppose we have the following table – that is called employee. The employee table has 2 columns – one for the employee name (called employee_name), and one for the employee location (called employee_location). Joe New York Sunil India Alex Russia Albert Canada Jack New York Now, suppose we want to find out which employees are from the same location as the employee named Joe. In this example, that location would be New York. Let’s assume – for the sake of our example – that we can not just directly search the table for people who live in New York with a simple query like this (maybe because we don’t want to hardcode the city name) in the SQL query. SELECT employee_name. WHERE employee_location = "New York". So, instead of a query like that what we could do is write a nested SQL query (basically a query within another query – which is more commonly called a subquery) like this. SELECT employee_name. WHERE employee_location in. SELECT employee_location. WHERE employee_name = "Joe"). A subquery is inefficient Using a subquery for such a simple question is inefficient. Is there a more efficient and elegant solution to this problem? It turns out that there is a more efficient solution – we can use something called a self join. A self join is basically when a table is joined to itself. The way you should visualize a self join for a given table is by imagining that a join is performed between two identical copies of that table. And that is exactly why it is called a self join – because of the fact that it’s just the same table being joined to another copy of itself rather than being joined with a different table. How does a self join work. Before we come up with a solution for this problem using a self join, we should go over some concepts so that you can fully understand how a self join works. This will also make the SQL in our self join tutorial a lot easier to understand, which you will see further below. A self join must have aliases. In a self join we are joining the same table to itself by essentially creating two copies of that table. But, how do we distinguish between the two different copies of the table – because there is only one table name after all? Well, when we do a self join, the table names absolutely must use aliases otherwise the column names would be ambiguous. In other words, we would not know which of the two copies of the table’s columns is being referenced without using an alias for each copy of the table. If you don’t already know what an alias is, it’s simply another name given to a table – think of an alias as a nickname – and that nickname is then used in the SQL query to reference the table. Because we need two copies of the employee table, we will just use the aliases e. Self join predicate. As with any join there must be a condition upon which a self join is performed – we can not just arbitrarily say “do a self join”, without specifying some condition. That condition will be our join predicate. If you need a refresher on join predicates (or just joins in general) then check this link out: Inner vs. Outer joins. Now, let’s come up with a solution to the original problem using a self join instead of a subquery. This will help illustrate how exactly a self join works. The key question that we must ask ourselves is what should our join predicate be in this example? Well, we want to find all the employees who have the same location as Joe. Because we want to match between our two tables (both of which are the same table – employee – aliased as e. WHERE e. 1. employee_location = e. But is that enough to give us what we want? No, it’s not, because we also want to filter the rows returned since we only want people who are from the same location as Joe. So, how can we filter the rows returned so that only people from Joe’s location are returned? Well, what we can do is simply add a condition on one of the tables (e. Joe. Then, the other table (e. WHERE e. 1. employee_location = e. We will then just select the names from e. Joe’s name. If that’s confusing then keep reading further to understand more about how the query will work. So, the self join query that we come up with looks like this. Self Join SQL Example. SELECT e. 1. employee_name. FROM employee e. 1, employee e. WHERE e. 1. employee_location = e. AND e. 2. employee_name="Joe". This query will return the names Joe and Jack – since Jack is the only other person who lives in New York like Joe. Generally, queries that refer to the same table can be greatly simplified by re- writing the queries as self joins. And, there is definitely a performance benefit for this as well. What does a self join look like? It will help tremendously to actually visualize the actual results of a self join internally. Remember that a self join is just like any other join, where the two tables are merged into one temporary table. First off, you should visualize that we have two separate copies of the employee table, which are given aliases of e. These copies would simply look like this – note that we shortened the column names from employee_name and employee_location to just Name and Location for convenience. Name. Location Joe New York Sunil India Alex Russia Albert Canada Jack New York Name. Location Joe New York Sunil India Alex Russia Albert Canada Jack New York. And the final results of running the self join query above – the actual joined table – would look like this. Joe. New York. Joe. New York. Jack. New York. Joe. New York. Self joins versus inner joins. Are self joins and inner joins the same? You might be wondering if all self joins are also inner joins. After all, in our example above our self join uses an inner join because only the rows that match based on the join predicate are returned – non- matching rows are not returned. Well, it turns out that a self join and inner join are completely different concepts. A self join could just as well be an outer join or an inner join – it just depends on how the query is written. We could easily change the query we used above to do a LEFT OUTER JOIN – while the query still remains a self join – but that wouldn’t give us the results we want in our example. So, we use an implied inner join instead because that gives us the correct results. Remember that a query is a self join as long as the two tables being joined are exactly the same table, but whether it’s an inner join or outer join depends on what is specified in the SQL. And, inner/outer joins are separate concepts entirely from a self join. Self joins manager employee example. The most commonly used example for self joins is the classic employee manager table. The table is called Employee, but holds all employees – including their managers. Every employee has an ID, and there is also a column for the manager ID. So, for example, let’s say we have a table that looks like this – and we call it Employee. Employee. IDName. Manager. ID1. Sam. Harry. 44. Manager. NULL1. 0Another. Manager. NULL. Notice that in the table above there are two managers, conveniently named “Manager” and “Another. Manager”. And, those managers don’t have managers of their own – as noted by the NULL value in their Manager column. Now, given the table above, how can we return results that will show each employee’s name, and his/her manager’s name in nicely arranged results – with the employee in one column and his/her manager’s name in the other column. Well, it turns out we can use a self join to do this. Try to come up with the SQL on your own before reading our answer. Self join manager employee answer. In order to come up with a correct answer for this problem, our goal should be to perform a self join that will have both the employee information and manager information in one row. First off, since we are doing a self join, it helps to visualize the one table as two tables – let’s give them aliases of e. Now, with that in mind, we want the employee’s information on one side of the joined table and the manager’s information on the other side of the joined table. So, let’s just say that we want e. What should our join predicate be in that case? Well, the join predicate should look like “ON e. Manager. ID = e. 2. Employee. ID” – this basically says that we should join the two tables (a self join) based on the condition that the manager ID in e. ID in e. 2. In other words, an employee’s manager in e. An illustration will help clarify this.
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