Advance SQL: SQL Performance Tuning. SQL Views

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1 Advance SQL: SQL Performance Tuning SQL Views A view is nothing more than a SQL statement that is stored in the database with an associated name. A view is actually a composition of a table in the form of a predefined SQL query. A view can contain all rows of a table or select rows from a table. A view can be created from one or many tables which depends on the written SQL query to create a view. Views, which are kind of virtual tables, allow users to do the following: Structure data in a way that users or classes of users find natural or intuitive. Restrict access to the data such that a user can see and (sometimes) modify exactly what they need and no more. Summarize data from various tables which can be used to generate reports. SQL CREATE VIEW Statement In SQL, a view is a virtual table based on the result-set of an SQL statement. A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table. SQL CREATE VIEW Syntax CREATE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition Note: A view always shows up-to-date data! The database engine recreates the data, using the view's SQL statement, every time a user queries a view. SQL CREATE VIEW Examples If you have the Northwind database you can see that it has several views installed by default.

2 The view "Current Product List" lists all active products (products that are not discontinued) from the "Products" table. The view is created with the following SQL: CREATE VIEW [Current Product List] AS SELECT ProductID,ProductName FROM Products WHERE Discontinued=No We can query the view above as follows: SELECT * FROM [Current Product List] Another view in the Northwind sample database selects every product in the "Products" table with a unit price higher than the average unit price: CREATE VIEW [Products Above Average Price] AS SELECT ProductName,UnitPrice FROM Products WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products) We can query the view above as follows: SELECT * FROM [Products Above Average Price] Another view in the Northwind database calculates the total sale for each category in Note that this view selects its data from another view called "Product Sales for 1997": CREATE VIEW [Category Sales For 1997] AS SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales FROM [Product Sales for 1997] GROUP BY CategoryName We can query the view above as follows: SELECT * FROM [Category Sales For 1997] We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages": SELECT * FROM [Category Sales For 1997] WHERE CategoryName='Beverages'

3 WITH CHECK OPTION: The WITH CHECK OPTION is a CREATE VIEW statement option. The purpose of the WITH CHECK OPTION is to ensure that all UPDATE and INSERTs satisfy the condition(s) in the view definition. If they do not satisfy the condition(s), the UPDATE or INSERT returns an error. The following is an example of creating same view CUSTOMERS_VIEW with the WITH CHECK OPTION: CREATE VIEW CUSTOMERS_VIEW AS SELECT name, age FROM CUSTOMERS WHERE age IS NOT NULL WITH CHECK OPTION; The WITH CHECK OPTION in this case should deny the entry of any NULL values in the view's AGE column, because the view is defined by data that does not have a NULL value in the AGE column. SQL Updating a View You can update a view by using the following syntax: SQL CREATE OR REPLACE VIEW Syntax CREATE OR REPLACE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition Now we want to add the "Category" column to the "Current Product List" view. We will update the view with the following SQL:

4 CREATE OR REPLACE VIEW [Current Product List] AS SELECT ProductID,ProductName,Category FROM Products WHERE Discontinued=No Updating a View: A view can be updated under certain conditions: The SELECT clause may not contain the keyword DISTINCT. The SELECT clause may not contain summary functions. The SELECT clause may not contain set functions. The SELECT clause may not contain set operators. The SELECT clause may not contain an ORDER BY clause. The FROM clause may not contain multiple tables. The WHERE clause may not contain subqueries. The query may not contain GROUP BY or HAVING. Calculated columns may not be updated. All NOT NULL columns from the base table must be included in the view in order for the INSERT query to function. SQL > UPDATE CUSTOMERS_VIEW SET AGE = 35 WHERE name='ramesh'; Inserting Rows into a View: Rows of data can be inserted into a view. The same rules that apply to the UPDATE command also apply to the INSERT command. Here we can not insert rows in CUSTOMERS_VIEW because we have not included all the NOT NULL columns in this view, otherwise you can insert rows in a view in similar way as you insert them in a table.

5 Deleting Rows into a View: Rows of data can be deleted from a view. The same rules that apply to the UPDATE and INSERT commands apply to the DELETE command. Following is an example to delete a record having AGE= 22. SQL > DELETE FROM CUSTOMERS_VIEW WHERE age = 22; This would ultimately delete a row from the base table CUSTOMERS and same would reflect in the view itself. Now, try to query base table, and SELECT statement would produce the following result: ID NAME AGE ADDRESS SALARY Ramesh 35 Ahmedabad Khilan 25 Delhi kaushik 23 Kota Chaitali 25 Mumbai Hardik 27 Bhopal Muffy 24 Indore SQL Dropping a View You can delete a view with the DROP VIEW command. SQL DROP VIEW Syntax DROP VIEW view_name Read-Only View We can create a view with read-only option to restrict access to the view.

6 Syntax to create a view with Read-Only Access CREATE or REPLACE force view view_name AS SELECT column_name(s) FROM table_name WHERE condition with read-only The above syntax will create view for read-only purpose, we cannot Update or Insert data into read-only view. It will throw an error. Types of View There are two types of view, Simple View Complex View Simple View Complex View Created from one table Created from one or more table Does not contain functions Contain functions Does not contain groups of data Contains groups of data SQL Joins SQL joins are used to combine rows from two or more tables.

7 An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them. The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN returns all rows from multiple tables where the join condition is met. Let's look at a selection from the "Orders" table: OrderID CustomerID OrderDate Then, have a look at a selection from the "Customers" table: CustomerID CustomerName ContactName Country 1 Alfreds Futterkiste Maria Anders Germany 2 Ana Trujillo Emparedados y helados Ana Trujillo Mexico 3 Antonio Moreno Taquería Antonio Moreno Mexico

8 Example SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate FROM Orders INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID; it will produce something like this: OrderID CustomerName OrderDate Ana Trujillo Emparedados y helados 9/18/ Antonio Moreno Taquería 11/27/ Around the Horn 12/16/ Around the Horn 11/15/ Berglunds snabbköp 8/12/1996 Different SQL JOINs Before we continue with examples, we will list the types of the different SQL JOINs you can use: INNER JOIN: Returns all rows when there is at least one match in BOTH tables LEFT JOIN: Return all rows from the left table, and the matched rows from the right table RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table FULL JOIN: Return all rows when there is a match in ONE of the tables

9 SQL INNER JOIN Keyword The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables. SQL INNER JOIN Syntax SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name=table2.column_name; or: SELECT column_name(s) FROM table1 JOIN table2 ON table1.column_name=table2.column_name; PS! INNER JOIN is the same as JOIN. SQL INNER JOIN Example The following SQL statement will return all customers with orders: Example SELECT Customers.CustomerName, Orders.OrderID FROM Customers INNER JOIN Orders ON Customers.CustomerID=Orders.CustomerID ORDER BY Customers.CustomerName;

10 Note: The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns. If there are rows in the "Customers" table that do not have matches in "Orders", these customers will NOT be listed. SQL LEFT JOIN Keyword The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL in the right side when there is no match. SQL LEFT JOIN Syntax SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name=table2.column_name; or: SELECT column_name(s) FROM table1 LEFT OUTER JOIN table2 ON table1.column_name=table2.column_name; PS! In some databases LEFT JOIN is called LEFT OUTER JOIN. SQL LEFT JOIN Example The following SQL statement will return all customers, and any orders they might have: Example SELECT Customers.CustomerName, Orders.OrderID FROM Customers LEFT JOIN Orders

11 ON Customers.CustomerID=Orders.CustomerID ORDER BY Customers.CustomerName; SQL RIGHT JOIN Keyword The RIGHT JOIN keyword returns all rows from the right table (table2), with the matching rows in the left table (table1). The result is NULL in the left side when there is no match. SQL RIGHT JOIN Syntax SELECT column_name(s) FROM table1 RIGHT JOIN table2 ON table1.column_name=table2.column_name; or: SELECT column_name(s) FROM table1 RIGHT OUTER JOIN table2 ON table1.column_name=table2.column_name; PS! In some databases RIGHT JOIN is called RIGHT OUTER JOIN. SQL RIGHT JOIN Example The following SQL statement will return all employees, and any orders they have placed: Example SELECT Orders.OrderID, Employees.FirstName FROM Orders RIGHT JOIN Employees ON Orders.EmployeeID=Employees.EmployeeID ORDER BY Orders.OrderID;

12 SQL FULL OUTER JOIN Keyword The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2). The FULL OUTER JOIN keyword combines the result of both LEFT and RIGHT joins. SQL FULL OUTER JOIN Syntax SELECT column_name(s) FROM table1 FULL OUTER JOIN table2 ON table1.column_name=table2.column_name; SQL FULL OUTER JOIN Example The following SQL statement selects all customers, and all orders: SELECT Customers.CustomerName, Orders.OrderID FROM Customers FULL OUTER JOIN Orders ON Customers.CustomerID=Orders.CustomerID ORDER BY Customers.CustomerName; SQL Sequence Sequence is a feature supported by some database systems to produce unique values on demand. Some DBMS like MySQL supports AUTO_INCREMENT in place of Sequence. AUTO_INCREMENT is applied on columns, it automatically increments the column value by 1 each time a new record is entered into the table. Sequence is also some what similar to AUTO_INCREMENT but its has some extra features.

13 Syntax to create sequences is, CREATE Sequence sequence-name start with initial-value increment by increment-value maxvalue maximum-value cycle nocycle initial-value specifies the starting value of the Sequence, increment-value is the value by which sequence will be incremented and maxvalue specifies the maximum value until which sequence will increment itself.cycle specifies that if the maximum value exceeds the set limit, sequence will restart its cycle from the begining. No cycle specifies that if sequence exceeds maxvalue an error will be thrown. Example to create Sequence The sequence query is following CREATE Sequence seq_1 start with 1 increment by 1 maxvalue 999 cycle ; Example to use Sequence The class table, ID NAME 1 abhi 2 adam

14 4 alex The sql query will be, INSERT into class value(seq_1.nextval,'anu'); Result table will look like, ID NAME 1 abhi 2 adam 4 alex 1 anu Once you use nextval the sequence will increment even if you don't Insert any record into the table. ALTER SEQUENCE Purpose Use the ALTER SEQUENCE statement to change the increment, minimum and maximum values, cached numbers, and behavior of an existing sequence. This statement affects only future sequence numbers. Prerequisites The sequence must be in your own schema, or you must have the ALTER object privilege on the sequence, or you must have the ALTER ANYSEQUENCE system privilege.

15 Syntax alter_sequence::= Examples Modifying a Sequence: Examples This statement sets a new maximum value for the customers_seq sequence, which was created in "Creating a Sequence: Example": CREATE SEQUENCE customers_seq START WITH 1000 INCREMENT BY 1 NOCACHE NOCYCLE; ALTER SEQUENCE customers_seq MAXVALUE 1500; This statement turns on CYCLE and CACHE for the customers_seq sequence: ALTER SEQUENCE customers_seq CYCLE CACHE 5; Example CREATE SCHEMA Test ; GO CREATE SEQUENCE Test.TestSeq

16 AS int START WITH 125 INCREMENT BY 25 MINVALUE 100 MAXVALUE 200 CYCLE CACHE 3 ; GO And Alter ALTER SEQUENCE Test. TestSeq RESTART WITH 100 INCREMENT BY 50 MINVALUE 50 MAXVALUE 200 NO CYCLE NO CACHE ; GO DROP SEQUENCE Purpose Use the DROP SEQUENCE statement to remove a sequence from the database. Prerequisites The sequence must be in your own schema or you must have the DROP ANY SEQUENCE system privilege. Syntax drop_sequence::= Example DROP SEQUENCE oe.customers_seq; SQL CREATE INDEX Statement The CREATE INDEX statement is used to create indexes in tables. Indexes allow the database application to find data fast; without reading the whole table.

17 Indexes An index can be created in a table to find data more quickly and efficiently. The users cannot see the indexes, they are just used to speed up searches/queries. The CREATE INDEX Command: The basic syntax of CREATE INDEX is as follows: CREATE INDEX index_name ON table_name; Single-Column Indexes: A single-column index is one that is created based on only one table column. The basic syntax is as follows: CREATE INDEX index_name ON table_name (column_name); Unique Indexes: Unique indexes are used not only for performance, but also for data integrity. A unique index does not allow any duplicate values to be inserted into the table. The basic syntax is as follows: CREATE UNIQUE INDEX index_name on table_name (column_name); Composite Indexes: A composite index is an index on two or more columns of a table. The basic syntax is as follows: CREATE INDEX index_name on table_name (column1, column2); Whether to create a single-column index or a composite index, take into consideration the column(s) that you may use very frequently in a query's WHERE clause as filter conditions.

18 Should there be only one column used, a single-column index should be the choice. Should there be two or more columns that are frequently used in the WHERE clause as filters, the composite index would be the best choice. Implicit Indexes: Implicit indexes are indexes that are automatically created by the database server when an object is created. Indexes are automatically created for primary key constraints and unique constraints. The DROP INDEX Command: An index can be dropped using SQL DROP command. Care should be taken when dropping an index because performance may be slowed or improved. The basic syntax is as follows: DROP INDEX index_name; You can check INDEX Constraint chapter to see actual examples on Indexes. When should indexes be avoided? Although indexes are intended to enhance a database's performance, there are times when they should be avoided. The following guidelines indicate when the use of an index should be reconsidered: Indexes should not be used on small tables. Tables that have frequent, large batch update or insert operations. Indexes should not be used on columns that contain a high number of NULL values. Columns that are frequently manipulated should not be indexed. Creating Snapshots You create a snapshot using the SQL command CREATE SNAPSHOT. As when creating tables, you can specify storage characteristics, extent sizes and allocation, and the tablespace to hold the snapshot, or a cluster to hold the snapshot (in which case all of the previous options do not apply). You can also specify how the snapshot is to be refreshed and the distributed query that defines the snapshot; this is unique to snapshots.

19 For example, the following CREATE SNAPSHOT statement defines a local snapshot to replicate the remote EMP table located in NY: CREATE SNAPSHOT emp_sf PCTFREE 5 PCTUSED 60 TABLESPACE users STORAGE (INITIAL 50K NEXT 50K PCTINCREASE 50) REFRESH FAST START WITH sysdate NEXT sysdate + 7 AS SELECT * FROM scott.emp@sales.ny.com; Whenever you create a snapshot, Oracle immediately fills the base table with the rows returned by the query that defines the snapshot. Thereafter, the snapshot is refreshed as specified by the REFRESH clause; see "Refreshing Snapshots" Creating a Clustered Snapshot You can create a snapshot in a cluster, just as you can a table. For example, the following statement creates a snapshot named EMP_DALLAS in the EMP_DEPT cluster: CREATE SNAPSHOT emp_dallas... CLUSTER emp_dept... ; You can change a snapshot's storage parameters using the ALTER SNAPSHOT command. For example, the following command alters the EMP snapshot's PCTFREE parameter: ALTER SNAPSHOT emp PCTFREE 10; You cannot change a snapshot's defining query; you must drop the snapshot and then re-create it. Privileges Required to Alter a Snapshot To alter a snapshot's storage parameters, the snapshot must be contained in your schema or you must have the ALTER ANY SNAPSHOT and ALTER ANY TABLE system privileges. ALTER SNAPSHOT - Examples The only parts of a snapshot that can be altered are its storage parameters, refresh type and refresh start, and next interval. The select for the snapshot, base tables, and other data related items cannot be changed without dropping and recreating the snapshot. For example, to alter the PCTFREE and PCTUSED of the trial_summary snapshot the following command would be used: SQL> ALTER SNAPSHOT trial_summary 1: PCTFREE 10 PCTUSED 80;

20 snapshot altered If we wished to change the refresh interval to THURSDAY from FRIDAY at 1900 hours the command would be: SQL> ALTER SNAPSHOT trial_summary 1: NEXT NEXT_DAY(SYSDATE, 'THURSDAY') + 19/24; snapshot altered To alter the refresh option used by the LOCAL_TRANSACTION snapshot, execute the comand: ALTER SNAPSHOT local_transaction COMPLETE; Dropping Snapshots You can drop a snapshot independently of its master tables or the snapshot log. To drop a local snapshot, use the SQL command DROP SNAPSHOT. For example: DROP SNAPSHOT emp; If you drop the only snapshot of a master table, you should also drop the snapshot log of the master table, if there is one. Privileges Required to Drop a Snapshot Only the owner of a snapshot or a user with the DROP ANY SNAPSHOT system privilege can drop a snapshot ORACLE/PLSQL: SYNONYMS This Oracle tutorial explains how to create and drop synonyms in Oracle with syntax and examples. DESCRIPTION A synonym is an alternative name for objects such as tables, views, sequences, stored procedures, and other database objects. You generally use synonyms when you are granting access to an object from another schema and you don't want the users to have to worry about knowing which schema owns the object.

21 CREATE SYNONYM (OR REPLACE) You may wish to create a synonym so that users do not have to prefix the table name with the schema name when using the table in a query. Syntax The syntax to create a synonym in Oracle is: CREATE [OR REPLACE] [PUBLIC] SYNONYM [schema.] synonym_name FOR [schema.] object_name [@ dblink]; OR REPLACE PUBLIC schema object_name Allows you to recreate the synonym (if it already exists) without having to issue a DROP synonym command. It means that the synonym is a public synonym and is accessible to all users. Remember though that the user must first have the appropriate privileges to the object to use the synonym. The appropriate schema. If this phrase is omitted, Oracle assumes that you are referring to your own schema. The name of the object for which you are creating the synonym. It can be one of the following: table view sequence stored procedure function package materialized view java class schema object user-defined object synonym

22 Example Let's look at an example of how to create a synonym in Oracle. For example: CREATE PUBLIC SYNONYM suppliers FOR app.suppliers; This first CREATE SYNONYM example demonstrates how to create a synonym called suppliers. Now, users of other schemas can reference the table called suppliers without having to prefix the table name with the schema named app. For example: SELECT * FROM suppliers; If this synonym already existed and you wanted to redefine it, you could always use the OR REPLACE phrase as follows: CREATE OR REPLACE PUBLIC SYNONYM suppliers FOR app.suppliers; DROP SYNONYM Once a synonym has been created in Oracle, you might at some point need to drop the synonym. Syntax The syntax to drop a synonym in Oracle is: DROP [PUBLIC] SYNONYM [schema.] synonym_name [force]; PUBLIC force Allows you to drop a public synonym. If you have specified PUBLIC, then you don't specify a schema. It will force Oracle to drop the synonym even if it has dependencies. It is probably not a good idea to use force as it can cause invalidation of Oracle objects.

23 Example Let's look at an example of how to drop a synonym in Oracle. For example: DROP PUBLIC SYNONYM suppliers; This DROP statement would drop the synonym called suppliers that we defined earlier.