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CS 430Database Theory

Winter 2005

Lecture 14: Additional SQL Topics

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Additional Topics

Views Programmatic Interface

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Views

Views are Virtual tables They can be referenced in SELECT statements just

like another table They can be referenced in INSERT, UPDATE, and

DELETE when that makes sense Why Views?

Information hiding, etc. etc. Integration with Privileges

E.g. have SELECT privilege on view, not underlying table

MySQL: Views 5.0 and later

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Defining a View

CREATE VIEW view_name(col1, …)

AS SELECT … ; Column names are optional

Column names are taken from SELECT if not specified

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View Update

Problematic Not all views are updateable

Aggregate columns Joins

Simple Example: CREATE VIEW Emp_Dept

AS SELECT SSN, Dno, DNameFROM Employee JOIN Department ON Dno=Dnumber;

UPDATE Emp_DeptSET DName = ‘New Research’WHERE SSN = ‘333445555’;

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More View Update

Safe (all DBMSs support) A View which is a Row and Column subset of a

single table is updateable INSERTS require default values for invisible column

Check Option CREATE VIEW … WITH CHECK OPTION; For updateable views All updates will appear in the view

Good practice: Specify for views you intend to be updateable

DBMS will tell you if view is not updateable

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How are Views Implemented? Query Modification:

Modify a query on the view to be a query on the underlying tables

Simple query on a complex view becomes a complex query View Materialization

Maintain a table with the contents of the view Mapping database updates to changes in the view is a

problem Some DBMSs allow the DBA to choose a strategy

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Application Interface

Interactive SQL: Type SQL and have it directly executed Good for: Database Administration, one time

and/or ad hoc queries, shell scripts Not good for applications

Application Interface Techniques Application Programming Interface Embedded SQL Database Programming Language

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Impedance Mismatch

Problem with host languages and SQL: Mapping needed between host language types and SQL

data types Databases have Rows, Tables or Row Sets

Host languages have their own types of composite data types Often would like to map between host language objects

and rows in tables in the DBMS Dealing with update can get tricky

Bottom line None of these problems is close to unsolvable Code is required to deal with all of these problems

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Application Interface Concepts Connection

A connection to the database Multiple SQL statements can be processed using

a connection Host, username, password, etc. specified when

connection is established First step in interacting with the database is to

establish a connection

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Application Interface Concepts Cursor

SELECT can return multiple rows. How do I handle this in my host language?

Solution: Create a Cursor Reflects the current status of the query Allows iteration over the results of the query

FETCH fetches the next row from the Cursor Some systems allow backward iteration and

seeking on a cursor

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Application Interface Concepts Single Statement SELECT

When a SELECT will return a single row, directly FETCH the results of the select in a single statement

Shared variables Variables that are shared between the database

interface (SQL) and the programming language Alternatively, bindings between variables in SQL

statements and variables in the programming language

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Application Interface Concepts “Dynamic” SQL

SQL statements that are created on the fly by the program PREPARE

A statement is prepared prior to being executed Can be used to save overhead of repeated parsing

IMMEDIATE Execute the statement without prior preparation

SQLCODE, SQLSTATE Variables used to communicate errors, exceptions, state

between DBMS interface and program

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Embedded SQL

SQL statements are embedded into the host programming language

Precompiler or preprocessor used to convert the program into standard language Statements replaced by appropriate functions

calls See examples:

C: Figures 9.2, 9.3, 9.4, 9.5 in text Java: SQL/J: Figures 9.6, 9.7, 9.8, 9.9, 9.10

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Embedded SQL

Why? Integrated syntax for including SQL in application

programs Why not?

Integrated? Syntax for including SQL in application programs

Support is spotty Preprocessor is a mini-compiler

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Application Programming Interface API for accessing the database

Standard: SQL/CLI (Call Level Interface) Standardized version of ODBC

DBMS Specific DBMS specific API

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Application Programming Interface SQL statements represented as character

strings Some form of variable substitution needed Warning: May have to escape strings

Make function calls as appropriate Follow same model as Embedded SQL:

Establish a connection Open a cursor Execute statement Fetch results

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Example MySQL/Python

dno = raw_input('Enter a Department Number ... ').strip();connection = MySQLdb.connect(host = ‘localhost' db='example', user='example', passwd='example');try: cursor = connection.cursor() cursor.execute( ''' select ssn, fname, minit, lname, salary from employee where DNO = %(dno)s''', {'dno' : dno}) rows = cursor.fetchall() for row in rows: print 'SSN: %s, Name: %s %s %s, Salary: %s' % row cursor.close()finally: connection.close()

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Open Database Connectivity (ODBC) Originated by Microsoft

Standardized as the SQL/CLI

Connects to Data Sources rather than a specific database Initialization parameters

(.INI) describe data source

Can manage multiple data sources Picture courtesy MySQL Documentation

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Java Database Connectivity (JDBC) Same idea as ODBC, specialized for Java

Some changes to interface to adapt to Java style OO programming and strong typing

Provides same capabilities as ODBC

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ODBC Examples

C/ODBC Figures 9.11, 9.12 from book

Java/JDBC Figures 9.13, 9.14 from book

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Database Programming Language Standard form: SQL/PSM (Persistent Stored

Modules) Full scale programming language

Conditions, loops, exceptions, procedures and functions SQL statements SQL Data Types (e.g. Cursors)

Used to support triggers inside DBMS May or may not have API support for calling PSM

programs Short example: Figure 9.15 in text