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WITH , , select from
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v1 AS (select ... from ...) v2 AS (select ... from ...) v3 AS ... ... ...
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There are several advantages to using factored subqueries. First, they can make development easier by isolating each query (as I will show in Listing 9-18). Second, they make the code clearer. Using the previous example would look as follows: select ... from (select ... from (select ... from (select ... from ...) v3 ) v2 ) v1 When there is a problem with the query, it can be difficult to locate the actual problem. By using subquery factoring, you can create the subquery as a standalone query, then make it a factored subquery using WITH, SELECT * from it to check for completeness, and add in additional predicates, data transformations, exclude columns, and so on. If this query is also meant to be a factored subquery, you can then name it and SELECT * from it to check for completeness, add in additional predicates, and so on. Listing 9-18 shows how a statement using a factored subquery can be developed using a 3 step process. Each step in Listing 9-18 is executed separately. Listing 9-18. WITH Clause Development Example select , from group by x.deptno avg(x.msal) avg_sal employees x x.deptno; x.deptno avg(x.msal) avg_sal employees x x.deptno)
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WITH g AS (select , from group by select * from g;
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WITH g AS (select x.deptno , avg(x.msal) avg_sal from employees x group by x.deptno) select e.ename, e.init, e.msal from employees e join g using (deptno) where e.msal > g.avg_sal;
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If you define multiple subqueries in the WITH clause, you are allowed to refer to any subquery name that you defined earlier in the same WITH clause; that is, the definition of subquery V2 can refer to V1 in its FROM clause, and the definition of V3 can refer to both V1 and V2, as in the following: WITH v1 AS (select ... from ...) v2 AS (select ... from V1) , v3 AS (select ... from V2 join V1) select ... from ... , Under the hood, the Oracle DBMS has two ways to execute queries with a WITH clause: Merge the subquery definitions into the main query. This makes the subqueries behave just like inline views. Execute the subqueries, store the results in a temporary structure, and access the temporary structures from the main query.
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See Oracle SQL Reference for more details and examples on the WITH clause and subquery factoring.
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9.5 Hierarchical Queries
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Relational tables are flat structures. All rows of a table are equally important, and the order in which the rows are stored is irrelevant. However, some data structures have hierarchical relationships. A famous example in most books about relational database design is the bill of materials (BOM) problem, where you are supposed to design an efficient relational database structure to store facts about which (sub)components are needed to build more complicated components, up to highly complicated objects such as cars and airplanes. Figure 9-1 shows an ERM diagram with a typical solution. On the left, you see the most generic solution with a many-to-many relationship, and on the right you see a typical solution using two entities.
Figure 9-1. A solution for the bill of materials problem Notice that for the solution on the left-hand side, if you replaced the entity name PART with THING, and you replaced the two relationship descriptions with related to, then you would have the ultimate in generic data models! Although this book is not about database design, consider this joke as a serious warning: don t make your data models overly generic. Even if hierarchical data structures are correctly translated into relational tables, the retrieval of such structures can still be quite challenging. We have an example of a simple hierarchical relationship
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in our sample tables: the management structure in the EMPLOYEES table is implemented with the MGR column and its foreign key constraint to the EMPNO column of the same table.
Note In hierarchical structures, it is common practice to refer to parent rows and children rows. Another common (and self-explanatory) terminology is using a tree metaphor by referring to root, branch, and leaf rows.
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