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Updating Views and Answering Queries with Views

This lecture covers how to insert tuples into non-existent tables, reusing materialized views, and answering queries using views. It also discusses constraints in SQL, including keys, foreign keys, and referential integrity constraints.

marilynwilburn
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Updating Views and Answering Queries with Views

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  1. Lecture 6: Views Friday, January 17th, 2003

  2. Updating Views How can I insert a tuple into a table that doesn’t exist? Employee(ssn, name, department, project, salary) CREATE VIEW Developers AS SELECT name, project FROM Employee WHERE department = “Development” If we make the following insertion: INSERT INTO Developers VALUES(“Joe”, “Optimizer”) INSERT INTO Employee VALUES(NULL, “Joe”, NULL, “Optimizer”, NULL) It becomes:

  3. Non-Updatable Views CREATE VIEW Seattle-view AS SELECT seller, product, store FROM Person, Purchase WHERE Person.city = “Seattle” AND Person.name = Purchase.buyer How can we add the following tuple to the view? (“Joe”, “Shoe Model 12345”, “Nine West”) We need to add “Joe” to Person first, but we don’t have all its attributes

  4. Answering Queries Using Views • What if we want to use a set of views to answer a query. • Why? • The obvious reason… • Answering queries over web data sources. • Very cool stuff! (i.e., I did a lot of research on this).

  5. Reusing a Materialized View • Suppose I have only the result of SeattleView: SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = ‘Seattle’ AND Person.per-name = Purchase.buyer • and I want to answer the query SELECT buyer, seller FROM Person, Purchase WHERE Person.city = ‘Seattle’ AND Person.per-name = Purchase.buyer AND Purchase.product=‘gizmo’. Then, I can rewrite the query using the view.

  6. Query Rewriting Using Views Rewritten query: SELECT buyer, seller FROM SeattleView WHERE product= ‘gizmo’ Original query: SELECT buyer, seller FROM Person, Purchase WHERE Person.city = ‘Seattle’ AND Person.per-name = Purchase.buyer AND Purchase.product=‘gizmo’.

  7. Another Example • I still have only the result of SeattleView: SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = ‘Seattle’ AND Person.per-name = Purchase.buyer • but I want to answer the query SELECT buyer, seller FROM Person, Purchase WHERE Person.city = ‘Seattle’ AND Person.per-name = Purchase.buyer AND Person.Phone LIKE ‘206 543 %’.

  8. And Now? • I still have only the result of SeattleView: SELECT buyer, seller, product, store FROM Person, Purchase, Product WHERE Person.city = ‘Seattle’ AND Person.per-name = Purchase.buyer AND Purchase.product = Product.name • but I want to answer the query SELECT buyer, seller FROM Person, Purchase WHERE Person.city = ‘Seattle’ AND Person.per-name = Purchase.buyer.

  9. And Now? • I still have only the result of: SELECT seller, buyer, Sum(Price) FROM Purchase WHERE Purchase.store = ‘The Bon’ Group By seller, buyer • but I want to answer the query SELECT seller, Sum(Price) FROM Purchase WHERE Person.store = ‘The Bon’ Group By seller And what if it’s the other way around?

  10. Finally… • I still have only the result of: SELECT seller, buyer, Count(*) FROM Purchase WHERE Purchase.store = ‘The Bon’ Group By seller, buyer • but I want to answer the query SELECT seller, Count(*) FROM Purchase WHERE Person.store = ‘The Bon’ Group By seller

  11. The General Problem • Given a set of views V1,…,Vn, and a query Q, can we answer Q using only the answers to V1,…,Vn? • Why do we care? • We can answer queries more efficiently. • We can query data sources on the WWW in a principled manner. • Many, many papers on this problem. • The best performing algorithm: The MiniCon Algorithm, (Pottinger & (Ha)Levy, 2000).

  12. Querying the WWW • Assume a virtual schema of the WWW, e.g., • Course(number, university, title, prof, quarter) • Every data source on the web contains the answer to a view over the virtual schema: UW database: SELECT number, title, prof FROM Course WHERE univ=‘UW’ AND quarter=‘2/02’ Stanford database: SELECT number, title, prof, quarter FROM Course WHERE univ=‘Stanford’ User query: find all professors who teach “database systems”

  13. Constraints in SQL • A constraint = a property that we’d like our database to hold • The system will enforce the constraint by taking some actions: • forbid an update • or perform compensating updates

  14. Constraints in SQL Constraints in SQL: • Keys, foreign keys • Attribute-level constraints • Tuple-level constraints • Global constraints: assertions The more complex the constraint, the harder it is to check and to enforce simplest Mostcomplex

  15. Keys CREATE TABLE Product ( name CHAR(30) PRIMARY KEY, category VARCHAR(20)) OR: CREATE TABLE Product ( name CHAR(30), category VARCHAR(20) PRIMARY KEY (name))

  16. CREATE TABLE Product ( name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (name, category)) Keys with Multiple Attributes

  17. Other Keys CREATE TABLE Product ( productID CHAR(10), name CHAR(30), category VARCHAR(20), price INT,PRIMARYKEY (productID), UNIQUE (name, category)) There is at most one PRIMARY KEY;there can be many UNIQUE

  18. Foreign Key Constraints Referentialintegrityconstraints CREATE TABLE Purchase ( prodName CHAR(30) REFERENCES Product(name), date DATETIME) prodName is a foreign key to Product(name)name must be a key in Product

  19. Product Purchase

  20. Foreign Key Constraints • OR • (name, category) must be a PRIMARY KEY CREATE TABLE Purchase ( prodName CHAR(30), category VARCHAR(20), date DATETIME, FOREIGNKEY (prodName, category) REFERENCES Product(name, category)

  21. What happens during updates ? Types of updates: • In Purchase: insert/update • In Product: delete/update Product Purchase

  22. What happens during updates ? • SQL has three policies for maintaining referential integrity: • Reject violating modifications (default) • Cascade: after a delete/update do a delete/update • Set-null set foreign-key field to NULL READING ASSIGNEMNT: 7.1.5, 7.1.6

  23. Constraints on Attributes and Tuples • Constraints on attributes: NOT NULL -- obvious meaning... CHECK condition -- any condition ! • Constraints on tuples CHECK condition

  24. Whatis the difference fromForeign-Key ? CREATE TABLE Purchase ( prodName CHAR(30) CHECK (prodName INSELECT Product.nameFROM Product), date DATETIME NOT NULL)

  25. General Assertions CREATE ASSERTION myAssert CHECKNOT EXISTS( SELECT Product.nameFROM Product, PurchaseWHERE Product.name = Purchase.prodNameGROUPBY Product.nameHAVING count(*) > 200)

  26. Final Comments on Constraints • Can give them names, and alter later • Read in the book !!! • We need to understand exactly when they are checked • We need to understand exactly what actions are taken if they fail

  27. Triggers in SQL • A trigger contains an event, a condition, an action. • Event = INSERT, DELETE, UPDATE • Condition = any WHERE condition (may refer to the old and the new values) • Action = more inserts, deletes, updates • Many, many more bells and whistles... • Read in the book (it only scratches the surface...)

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