Download Additional Topics

CS 430
Database Theory
Winter 2005
Lecture 14: Additional SQL Topics
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Additional Topics
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Views
Programmatic Interface
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Views
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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?
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Information hiding, etc. etc.
Integration with Privileges
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E.g. have SELECT privilege on view, not underlying table
MySQL: Views 5.0 and later
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Defining a View
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CREATE VIEW view_name(col1, …)
AS SELECT … ;
Column names are optional
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Column names are taken from SELECT if not
specified
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View Update
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Problematic
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Not all views are updateable
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Aggregate columns
Joins
Simple Example:
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CREATE VIEW Emp_Dept
AS SELECT SSN, Dno, DName
FROM Employee JOIN Department ON Dno=Dnumber;
UPDATE Emp_Dept
SET DName = ‘New Research’
WHERE SSN = ‘333445555’;
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More View Update
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Safe (all DBMSs support)
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A View which is a Row and Column subset of a
single table is updateable
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INSERTS require default values for invisible column
Check Option
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CREATE VIEW … WITH CHECK OPTION;
For updateable views
All updates will appear in the view
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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?
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Query Modification:
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View Materialization
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Modify a query on the view to be a query on the underlying
tables
Simple query on a complex view becomes a complex query
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
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Interactive SQL:
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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
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Application Programming Interface
Embedded SQL
Database Programming Language
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Impedance Mismatch
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Problem with host languages and SQL:
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Mapping needed between host language types and SQL
data types
Databases have Rows, Tables or Row Sets
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Often would like to map between host language objects
and rows in tables in the DBMS
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Host languages have their own types of composite data types
Dealing with update can get tricky
Bottom line
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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
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Connection
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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
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Cursor
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SELECT can return multiple rows. How do I
handle this in my host language?
Solution: Create a Cursor
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Reflects the current status of the query
Allows iteration over the results of the query
FETCH fetches the next row from the Cursor
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Some systems allow backward iteration and
seeking on a cursor
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Application Interface Concepts
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Single Statement SELECT
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When a SELECT will return a single row, directly
FETCH the results of the select in a single
statement
Shared variables
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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
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“Dynamic” SQL
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SQL statements that are created on the fly by the program
PREPARE
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IMMEDIATE
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A statement is prepared prior to being executed
Can be used to save overhead of repeated parsing
Execute the statement without prior preparation
SQLCODE, SQLSTATE
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Variables used to communicate errors, exceptions, state
between DBMS interface and program
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Embedded SQL
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SQL statements are embedded into the host
programming language
Precompiler or preprocessor used to convert
the program into standard language
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Statements replaced by appropriate functions
calls
See examples:
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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
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Why?
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Integrated syntax for including SQL in application
programs
Why not?
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Integrated? Syntax for including SQL in
application programs
Support is spotty
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Preprocessor is a mini-compiler
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Application Programming Interface
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API for accessing the database
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Standard: SQL/CLI (Call Level Interface)
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Standardized version of ODBC
DBMS Specific
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DBMS specific API
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Application Programming Interface
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SQL statements represented as character
strings
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Some form of variable substitution needed
Warning: May have to escape strings
Make function calls as appropriate
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Follow same model as Embedded SQL:
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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)
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Originated by Microsoft
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Connects to Data
Sources rather than a
specific database
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Standardized as the
SQL/CLI
Initialization parameters
(.INI) describe data
source
Can manage multiple
data sources
Picture courtesy MySQL Documentation
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Java Database Connectivity (JDBC)
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Same idea as ODBC, specialized for Java
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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
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C/ODBC
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Figures 9.11, 9.12 from book
Java/JDBC
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Figures 9.13, 9.14 from book
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Database Programming Language
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Standard form: SQL/PSM (Persistent Stored
Modules)
Full scale programming language
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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
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