Databases and SQL Databases for data storage and access The - PDF document

3/4/13 CS108 Lecture 18: Databases and SQL Databases for data storage and access The Structured Query Language Aaron Stevens 4 March 2013 Computer Science What You’ll Learn Today Computer Science  How does Facebook generate unique pages for each user?  How is data stored within a computer?  What is a database, and how does a database fit into an application?  What are the operations we can do with data stored in a database? 1

3/4/13 Storing Data: Main Memory Computer Science Memory is used to store programs and other data that are currently in use.  LOAD, STOR operations Advantage of memory: short access times  Read/write times in nanoseconds (10 -9 sec) Disadvantages of memory:  relatively expensive ($/byte)  “ volatile ” Storing Data: Secondary Storage Computer Science Secondary storage is used to store data for later use (examples: disks, CD, DVD).  Data is written from memory to disk.  When needed, data is read back into memory. 2

3/4/13 Secondary Storage Computer Science Advantages of secondary storage:  relatively inexpensive ($/byte)  not “ volatile ” Disadvantage of secondary storage: long access times  Read times in milliseconds (10 -3 sec).  in 10 ms, a modern CPU can perform millions of operations! Thus, it's important to minimize the number of times that the disk is accessed. Example: Facebook Profile Computer Science What info is on a Facebook profile page?  First Name  Activities  Last Name  Favorite Books  Email  Favorite Movies  Password  Favorite Music  Birthday  Favorite Quotes  Favorite TV shows  About me No matter which page you view, it has these same elements… These data are stored in a database table. 3

3/4/13 What ’ s a Table? Computer Science Each table (sub-collection) is a collection of records, and each record contains fields. Example: a profiles table The primary key is a field which uniquely identifies one record within a table. Consider this URL: http://www.facebook.com/profile.php?id=919184 Primary Key Computer Science The Primary Key:  uniquely identifies a record within a table  is an ideal search key  is a way to create relationships between different tables Consider this URL again... http://www.facebook.com/profile.php?id=919184 4

3/4/13 Example: Status Updates Computer Science Not all data fits neatly into the profile table. Consider status updates… A separate Status table tracks status updates for all users.  timestamp, userid, status Example: Status Table Computer Science Each status message is related to exactly one profile by the foreign key (the field called ‘ id ’ ). These ids are called a foreign key, because they are primary keys in another table. 5

3/4/13 Example: Friends Computer Science The Facebook “ friend ” relationship is created by an entry in a friend table. Each record has two user ids: These ids are called a foreign keys, because they are primary keys in another table. Databases Computer Science A database is:  a collection of data stored in a way to enable quick access (by primary key)  organized into related sub-collections called tables. Example: a mini facebook database:  A profiles table has records of each user  A status table has records of status messages  A friends table has records friend relationships Each table (sub-collection) is organized by records, and each record contains fields. 6

3/4/13 DBMS Computer Science A database is a collection of data (not software). A database management system (DBMS) is the software which manages a database. Functions of a DBMS: Efficient storage Providing a logical view of data (tables, records) Query processing Transaction management DBMS Computer Science Efficient storage and retrieval. Indexing enables locating a record by unique attribute, called a key.  Example: looking up stocks by their symbol . Logical representation - storage by record.  Example: update the record for ID=‘5’ . 7

3/4/13 Query Processing Computer Science A query language is used to access and modify the data. SQL (Structured Query Language) is the standard for relational databases. Many different database vendors support SQL:  Oracle, Sybase, IBM DB2, MS SQL Server, MS Access  MySQL, SQLite (free/open-source) Transaction Integrity Computer Science A transaction is an atomic sequence of operations that must complete together (or fail completely) but must not be half-done. Example: using the bank machine  Check available balance, dispense cash, update balance. DBMS guarantees transaction integrity: completion or failure of a the entire sequence of steps that make up a transaction. 8

3/4/13 Database Applications Computer Science End users rarely interact with a database directly. A database-enabled application allows the users to interact with the database without needing to know the query language. Structured Query Language Computer Science You’ll need to know some SQL to write database- enabled applications.  independent language -- syntax and semantics. SQL is comprised of 2 sub-languages:  Data Manipulation Language (DML): SELECT, INSERT, UPDATE, DELETE  Data Definition Language (DDL): CREATE TABLE, DROP TABLE, ALTER TABLE We’ll focus on the DML for interacting with records. 9

3/4/13 Using sqlClient Program Computer Science You may use the sqliteClient.py program to experiment with SQL statements: http://cs-webapps.bu.edu/cs108/util/sqlClient.py Example: Mini FB Database Computer Science Let ’ s consider just 2 tables: Notice that these tables share some data:  ID is a primary key in profiles , and a foreign key in status ; this enables cross-table relationships. 10

3/4/13 The SELECT Query Computer Science General form: SELECT <field1>, <fields2>, … FROM <table1>, <table2>, … [WHERE <field>=<value> … ] The minimal SELECT query requires only a list of fields (or * for all) and a single table. Example: SELECT * FROM profiles The SELECT Query (continued) Computer Science The SELECT query can specify conditions using the WHERE clause, which creates a more refined result set (e.g. only matching records are returned). Example: 11

3/4/13 SQL Comparison Operators Computer Science SQL WHERE clauses can support all of the usual comparison operations. Here are the SQL comparison operators. The SELECT Query (continued) Computer Science Multiple WHERE criteria can be joined together using the logical operators AND , OR and NOT . Example: 12

3/4/13 The SELECT Query (continued) Computer Science WHERE criteria can use wildcard comparisons as well, using the LIKE clause for near- matches. Example: The INSERT Query Computer Science General form: INSERT INTO <table> VALUES (<val1>, <val2>,<val3> … ) The INSERT query will insert a record into the table. It requires a list values – one value for each field in the record. Example: 13

3/4/13 The UPDATE Query Computer Science General form: UPDATE <table> SET <field1>=<value1>, <field2>=<value2> [WHERE <field>=<value>] Example: Always use a WHERE clause in an UPDATE! The DELETE Query Computer Science General form: DELETE FROM <table> [WHERE <field>=<value>] Example: The DELETE query is extremely dangerous. Always verify your criteria before deleting! Always use a WHERE clause in a DELETE! 14

3/4/13 What You Learned Today Computer Science  Databases persistence versus volatility.  DBMS: efficiency, logical view, query language, and transaction integrity.  Structured Query Language  SELECT, UPDATE, INSERT, DELETE  Constraints with WHERE clause Next Time: The Python DB API Computer Science Python defines a standard API (objects and methods) for interaction with databases.  No standard implementation of this interface.  3 rd party developers write their own libraries which conforms to the standard. We will be using 2 different DMBS in CS108:  The SQLite3 DBMS comes standard with Python  Free, nothing additional to install  We’ll move to the MySQL DBMS for web-application projects starting in 2 weeks 15

3/4/13 Using sqliteClient Program Computer Science You may use the sqliteClient.py program to experiment with SQL statements. It is accesible here: http://cs-webapps.bu.edu/cs108/util/sqlClient.py  Check your SQL statements against this client to rule out SQL syntax errors.  Then implement the SQL with parameterized data in your client program. Announcements and To Do Computer Science  Readings:  SQL Tutorial (today) http://www.firstsql.com/tutor.htm  Python DBAPI and sqlite3 (for Wednesday) http://docs.python.org/library/sqlite3.html 16

3/4/13 Facebook Haiku Computer Science Avoiding Facebook Became too much of a chore So I relented � - Scott M. Sokol Using Python with MS Access Computer Science Should you want to use Python with Microsoft Access, you will need to download and install 2 components: 1- install Mark Hammond's pywin32-210 for python 2.5 http://starship.python.net/crew/mhammond/win32/Downloads.html 2- download adodbapi.zip from http://adodbapi.sourceforge.net/ Unzip adodbapi.zip into C:\Python25\Lib\site-packages\ This is only available for Windows! 17