EGCI 321: Database Systems. Dr. Tanasanee Phienthrakul

Transcription

1 1 EGCI 321: Database Systems Dr. Tanasanee Phienthrakul

2 2 Chapter 10 Data Definition Language (DDL)

3 3 Basic SQL SQL language Considered one of the major reasons for the commercial success of relational databases SQL Structured Query Language Statements for data definitions, queries, and updates (both DDL and DML) Core specification Plus specialized extensions

4 4 SQL Data Definition and Data Types Terminology: Table, row, and column used for relational model terms relation, tuple, and attribute CREATE statement Main SQL command for data definition

5 5 Schema and Catalog Concepts in SQL SQL schema Identified by a schema name Includes an authorization identifier and descriptors for each element Schema elements include Tables, constraints, views, domains, and other constructs Each statement in SQL ends with a semicolon

6 6 The CREATE TABLE Command in SQL Specify a new relation Provide name Specify attributes and initial constraints Can optionally specify schema: CREATE TABLE COMPANY.EMPLOYEE... or CREATE TABLE EMPLOYEE...

7 7 The CREATE TABLE Command in SQL (cont.) Base tables (base relations) Relation and its tuples are actually created and stored as a file by the DBMS Virtual relations Created through the CREATE VIEW statement

8 8 Rational Model in SQL Creates the Students relation. Observe that the type (domain) of each field is specified, and enforced by the DBMS whenever tuples are added or modified. CREATE TABLE Students ( sid CHAR(20), name CHAR(20), login CHAR(10), age INTEGER, gpa REAL ) Another example: the Enrolled table holds information about courses that students take. CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2) )

9 9 Destroying Relations Destroys the relation Students. The schema information and the tuples are deleted. DROP TABLE Students

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12 12 Attribute Data Types and Domains in SQL Basic data types Numeric data types Integer numbers: INTEGER, INT, and SMALLINT Floating-point (real) numbers: FLOAT or REAL, and DOUBLE PRECISION Character-string data types Fixed length: CHAR(n), CHARACTER(n) Varying length: VARCHAR(n), CHAR VARYING(n), CHARACTER VARYING(n)

13 13 Attribute Data Types and Domains in SQL (cont.) Bit-string data types Fixed length: BIT(n) Varying length: BIT VARYING(n) Boolean data type Values of TRUE or FALSE or NULL DATE data type Ten positions Components are YEAR, MONTH, and DAY in the form YYYY- MM-DD

14 14 Attribute Data Types and Domains in SQL (cont.) Additional data types Timestamp data type (TIMESTAMP) Includes the DATE and TIME fields Plus a minimum of six positions for decimal fractions of seconds Optional WITH TIME ZONE qualifier INTERVAL data type Specifies a relative value that can be used to increment or decrement an absolute value of a date, time, or timestamp

15 15 Domain Attribute Data Types and Domains in SQL (cont.) Name used with the attribute specification Makes it easier to change the data type for a domain that is used by numerous attributes Improves schema readability Example: CREATE DOMAIN SSN_TYPE AS CHAR(9);

16 16 Specifying Constraints in SQL Basic constraints: Restrictions on attribute domains and NULLs Key and referential integrity constraints Constraints on individual tuples within a relation

17 17 NOT NULL Specifying Attribute Constraints and Attribute Defaults NULL is not permitted for a particular attribute Default value DEFAULT <value> CHECK clause Dnumber INT NOT NULL CHECK (Dnumber > 0 AND Dnumber < 21);

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19 19 Specifying Key and Referential Integrity Constraints PRIMARY KEY clause Specifies one or more attributes that make up the primary key of a relation Dnumber INT PRIMARY KEY; UNIQUE clause Specifies alternate (secondary) keys Dname VARCHAR(15) UNIQUE;

20 20 Primary and Candidate Keys in SQL Possibly many candidate keys (specified using UNIQUE), one of which is chosen as the primary key. For a given student and course, there is a single grade. CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid) ) Students can take only one course, and receive a single grade for that course; further, no two students in a course receive the same grade. CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade) ) Used carelessly, an IC can prevent the storage of database instances that arise in practice!

21 21 Specifying Key and Referential Integrity Constraints (cont.) FOREIGN KEY clause Default operation: reject update on violation Attach referential triggered action clause Options include SET NULL, CASCADE, and SET DEFAULT Action taken by the DBMS for SET NULL or SET DEFAULT is the same for both ON DELETE and ON UPDATE CASCADE option suitable for relationship relations

22 22 Foreign Keys in SQL Only students listed in the Students relation should be allowed to enroll for courses. CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students )

23 23 Referential Integrity in SQL CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students ON DELETE CASCADE ON UPDATE SET DEFAULT )

24 24 Giving Names to Constraints Keyword CONSTRAINT Name a constraint Useful for later altering

25 25 Specifying Constraints on Tuples Using CHECK CHECK clauses at the end of a CREATE TABLE statement Apply to each tuple individually CHECK (Dept_create_date <= Mgr_start_date); CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Age int CHECK (Age>=18) );

26 26 The CREATE TABLE Command in SQL (cont.) Some foreign keys may cause errors Specified either via: Circular references Or because they refer to a table that has not yet been created

27 27 Altering Relations The schema of Students is altered by adding a new field; every tuple in the current instance is extended with a null value in the new field. ALTER TABLE Students ADD firstyear integer ALTER TABLE Students DROP COLUMN firstyear ALTER TABLE Students MODIFY COLUMN firstyear char(20)

28 28 Entity sets to tables: CREATE TABLE Employees ( ssn CHAR(11), name CHAR(20), lot INTEGER, PRIMARY KEY (ssn) ) ER to Relational

29 29 Relationship Sets to Tables In translating a relationship set to a relation, attributes of the relation must include: Keys for each participating entity set (as foreign keys). This set of attributes forms a superkey for the relation. All descriptive attributes. CREATE TABLE Works_In ( ssn CHAR(1), ) did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments

30 30 Translating ER Diagrams with Key Constraints Map relationship to a table: Note that did is the key now! Separate tables for Employees and Departments. Since each department has a unique manager, we could instead combine Manages and Departments. CREATE TABLE Manages ( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments ) CREATE TABLE Dept_Mgr ( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees )