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Database Management Systems s-sql-ddl.pdf
- 1. SQL
- 2. SQL • SQL = “Structured Query Language” • Standard query language for relational DBMSs • History: Developed at IBM in late 70s 1st standard: SQL-86 2nd standard: SQL-92 3rd standard: SQL-99 or SQL3, well over 1000 pages “The nice thing about standards is that you have so many to choose from!” -Andrew S. Tannenbaum 2
- 3. SQL Consists of two parts: • Data Definition Language (DDL) Allows the specification of the database schema • Data Manipulation Language (DML) Allows the specification of queries & insert/update/delete statements 3
- 4. SQL data defini.on language
- 5. SQL Data Definition Language (DDL) 5 • Allows the specification of the database schema a set of relations with information about each relation • Schema information: - The schema of each relation - The domain of values associated with each attribute - Integrity constraints • Other information one can specify: - The set of indices to be maintained for each relation - Security and authorization information for each relation - The physical storage structure of each relation on disk
- 6. CREATE TABLE Command • Used to define a relation • Syntax: CREATE TABLE relationName (attrName1 Domain1, … attrNamen Domainn (integrity-constraint1), …, (integrity-constraintn)) • Example: CREATE TABLE branch (branch_name char(15) not null, branch_city char(30), assets integer) 6
- 7. Domain Types in SQL • char(n) Fixed length character string, with user-specified length n • varchar(n) Variable length character strings, with user-specified maximum length n • int Integer (a finite subset of integers that is machine-dependent) • smallint Small integer (a machine-dependent subset of the integer domain type) 7
- 8. Domain Types in SQL • numeric(p, d) Fixed point number, with user-specified precision of p digits, with d digits to the right of decimal point • real, double precision Floating point and double-precision floating point numbers, with machine-dependent precision • float Floating point number, with user-specified precision of at least n digits and others… 8
- 9. CREATE TABLE Command • Can be used to also specify: - Primary key attributes (PRIMARY KEY keyword) - Secondary keys (UNIQUE keyword) - Referential integrity constraints/foreign keys (FOREIGN KEY keyword) • Example: CREATE TABLE DEPT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9), MGRSTARTDATE CHAR(9), PRIMARY KEY (DNUMBER), UNIQUE (DNAME), FOREIGN KEY (MGRSSN) REFERENCES EMP ) 9 Primary key declaration on an attribute automatically ensures not null in SQL-92 onwards, but it needs to be explicitly stated in SQL-89
- 10. DROP TABLE Command • Used to remove a relation & its definition The relation can no longer be used in queries, updates, or any other commands since its description no longer exists • Syntax: DROP TABLE relationName • Example: DROP TABLE branch 10
- 11. 11 ALTER TABLE Command • Used to add/drop attributes from a relation • Add attribute syntax: ALTER TABLE relationName ADD attribName attribDomain All tuples in the relation are assigned null as the default value of the new attribute • Drop attribute syntax: ALTER TABLE relationName DROP attribName Dropping of attributes not supported by many DBMSs
- 12. ALTER TABLE Command • Since new attribute will have NULL values right after the ALTER command is executed, the NOT NULL constraint is not allowed for such an attribute • Example: ALTER TABLE employee ADD job varchar(12) • The database users must still enter a value for the new attribute JOB for each EMPLOYEE tuple. This can be done using the UPDATE command. 12
- 13. Integrity Constraints • Guard against accidental damage to the database by ensuring that authorized changes to the database do not result in a loss of data consistency. • Examples: - A savings account must have a balance greater than $10,000.00 - A salary of a bank employee must be at least $6.00 an hour - A customer must have a (non-null) phone number 13
- 14. SQL Integrity Constraints • On single relations: - not null - primary key - unique - check(P), where P is a predicate • On multiple relations: - foreign key 14
- 15. NOT NULL Constraint • Specifies that an attribute does not accept null values • Can be specified as part of: - The definition of an attribute in the CREATE TABLE statement e.g. CREATE TABLE branch (branch_name char(15) not null, …) - The definition of a domain (i.e., a “type” that can be used where a type is needed) e.g. CREATE DOMAIN Dollars numeric(12, 2) not null 15
- 16. UNIQUE Constraint • Specifies that a set of attributes form a candidate key • Syntax: UNIQUE (AttrName1, …, AttrNamen) • Candidate keys are permitted to be null (in contrast to primary keys) 16
- 17. CHECK Clause 17 • Enforce a predicate (condition) • Syntax: CHECK (Predicate) • Example: Ensure that the values of the assets are non-negative CREATE TABLE branch (branch_name char(15), branch_city char(30), assets integer, primary key (branch_name), CHECK (assets >= 0) )
- 18. CHECK Clause 18 • Can be also used to constrain domains e.g., CREATE DOMAIN hourly_wage numeric (5,2) CONSTRAINT value_test CHECK (value > = 4.00) • Can be named (useful to indicate which constraint an update violated) e.g., CREATE DOMAIN hourly_wage numeric (5,2) CONSTRAINT value_test CHECK (value > = 4.00)
- 19. Referential Integrity • Ensures that a value that appears in one relation for a given set of attributes also appears for a set of attributes in another relation. • Example: If “La Jolla” is a branch name appearing in one of the tuples in the account relation, then there exists a tuple in the branch relation for branch “La Jolla”. 19
- 20. Referential Integrity • In the CREATE TABLE statement we can use: - The PRIMARY KEY clause to list primary key (PK) attributes. - The UNIQUE KEY clause to list candidate key attributes - The FOREIGN KEY clause to list foreign key (FK) attributes and the name of the relation referenced by the FK. By default, a FK references PK attributes of the referenced table. 20
- 21. Referential Integrity Example 21 create table customer (customer_name char(20), customer_street char(30), customer_city char(30), primary key (customer_name )) create table branch (branch_name char(15), branch_city char(30), assets numeric(12,2), primary key (branch_name ))
- 22. Referential Integrity Example 22 create table account (account_number char(10), branch_name char(15), balance integer, primary key (account_number), foreign key (branch_name) references branch ) create table depositor (customer_name char(20), account_number char(10), primary key (customer_name, account_number), foreign key (account_number ) references account, foreign key (customer_name ) references customer )