Today s Topics... Homework Note. Lecture Notes CPSC 421 (Fall 2011) Intro to database systems (cont.) More on the relational model.

Transcription

1 Today s Topics... Intro to database systems (cont.) More on the relational model Intro to SQL Homework Note HW 1 is now due on Thursday (9/8) S. Bowers 1 of 13

2 Some Basic Terminology Database A database (DB) is a (structured) collection of persistent data In a relational DBMS, a database is a set of tables Database Management System A database management system (DBMS) is software that supports the definition, population, and query of databases MySQL, PostgreSQL, MS SQL Server, Oracle (Database), and IBM DB2 are some examples of relational DBMSs S. Bowers 2 of 13

3 Basic DBMS Architecture Web Forms Application Front Ends SQL Interface SQL Commands DBMS Plan Executor Operator Evaluator Parser Optimizer Query Evaluation Engine Transactio n Manager Lock Manager Concurrency Control File and Access Methods Buffer Manager Disk Space Manager Recovery Manager Index Files Data Files System Catalog Storage (on disk) We ll cover each these throughout the course S. Bowers 3 of 13

4 Anatomy of a Relation number owner balance type 101 J. Smith checking 102 W. Wei checking 103 J. Smith savings 104 M. Jones checking 105 H. Martin checking The relation schema includes The name of the relation ( ) The set of named attributes ( number, owner, balance, type ) Data types and constraints (more later) We often write the schema as: Account(Number, Owner, Balance, Type) The relation instance includes A set of rows (aka tuples or records ) Many instances may be possible for a given schema Informally the relation (schema + instance) is often called a table S. Bowers 4 of 13

5 Cardinality and Arity number owner balance type 101 J. Smith checking 102 W. Wei checking 103 J. Smith savings 104 M. Jones checking 105 H. Martin checking The arity (or degree ) of a relation is the number of attributes What is the arity of this table? The cardinality of a relation instance is the number of rows What is the cardinality of this table? S. Bowers 5 of 13

6 Keys number owner balance type 101 J. Smith checking 102 W. Wei checking 103 J. Smith savings 104 M. Jones checking 105 H. Martin checking deposit transaction id date amount /22/ /29/ /29/ /2/ check check number date amount /23/ /24/ A key is an attribute with unique values Each row in the table must have a unique key value Most tables have at least one key We typically underline the key attribute In these instances, which attributes are keys? That is, which attributes should be underlined? S. Bowers 6 of 13

7 Note that check numbers usually are not unique across s Thus, both and check number together should be the key A composite key has more than one attribute Note that designating an attribute as a key is a type of constraint The DBMS will not allow duplicate row values to be inserted! S. Bowers 7 of 13

8 Foreign Keys number owner balance type 101 J. Smith checking 102 W. Wei checking 103 J. Smith savings 104 M. Jones checking 105 H. Martin checking deposit transaction id date amount /22/ /29/ /29/ /2/ /5/ Should this row be legal? Account number 106 does not exist! How can we prevent it from happening? By using a foreign key constraint Make in deposit a foreign key that references number in Now, each deposit (row) must refer to an (row) If a DBMS enforces this constraint, we have referential integrity S. Bowers 8 of 13

9 Foreign Keys number owner balance type 101 J. Smith checking 102 W. Wei checking 103 J. Smith savings 104 M. Jones checking 105 H. Martin checking check check number date amount /23/ /24/ A foreign key is not required to be part of the key for a table This was the case for the deposit table ( not part of the key) In the check table, is a foreign key (and is part of the key) The target attribute(s) must be a key E.g., number in is the target key S. Bowers 9 of 13

10 Schema Version 1 number owner balance type deposit transaction id date amount check check number date amount Do we need to change this schema to allow multiple owners? If so, how? One approach would be to remove key constraint on.number This could introduce inconsistencies though (e.g., an with multiple different balances) Another approach would be to create a new owner table number owner balance type owner owner S. Bowers 10 of 13

11 Structured Query Language (SQL) The language used to talk to the DBMS SQL can be used for many operations... To create tables CREATE TABLE ( number INT NOT NULL, owner VARCHAR(50), balance FLOAT, type VARCHAR(8), PRIMARY KEY (number) ); To query the database SELECT * FROM WHERE type = "checking"; To insert rows into a table INSERT INTO VALUES (106, "M. Cruz", 10); And so on... S. Bowers 11 of 13

12 More on SQL SQL is a standard There have been a series: 1986, 1989, 1992 (SQL 2), (SQL:2008) See the Jim Melton podcast linked from website Even though it is a standard DBMS products differ in how much they support And many implement extra features (extensions) SQL is considered a declarative language In general, this means that you say what you want to happen Not how to perform it SQL is largely case insensitive Various conventions in use (lowercase, uppercase, camel case, etc.) Often keywords in uppercase, ids in lowercase Some systems allow for case-sensitive names S. Bowers 12 of 13

13 Query Example 1 deposit transaction id date amount /22/ /29/ /29/ /2/ SELECT, amount FROM deposit WHERE amount < 1000 The result of an SQL query is always a new table Often read queries inside out The FROM clause specifies what tables are being queried The WHERE clause is evaluated for each row in the table Which rows match the WHERE clause amount < 1000? Can think of this as creating an intermediate table The SELECT clause lists attributes to keep in the answer Here we only keep and amount Drop these from the intermediate table to get query answer Query answer: amount S. Bowers 13 of 13