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Chapter 8

Chapter 8. SQL: SchemaDefinition, Constraints, and Queries and Views. History of SQL. SQL: Structured Query Language In 1974, D. Chamberlin (IBM San Jose Laboratory) defined language called ‘Structured English Query Language’ (SEQUEL).

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Chapter 8

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  1. Chapter 8 SQL: SchemaDefinition, Constraints, and Queries and Views

  2. History of SQL • SQL: Structured Query Language • In 1974, D. Chamberlin (IBM San Jose Laboratory) defined language called ‘Structured English Query Language’ (SEQUEL). • A revised version, SEQUEL/2, was defined in 1976 but name was subsequently changed to SQL for legal reasons.

  3. History of SQL • Still pronounced ‘see-quel’, though official pronunciation is ‘S-Q-L’. • IBM subsequently produced a prototype DBMS called System R, based on SEQUEL/2.

  4. History of SQL • In late 70s, ORACLE appeared and was probably first commercial RDBMS based on SQL. • In 1987, ANSI and ISO published an initial standard for SQL. • In 1989, ISO published an addendum that defined an ‘Integrity Enhancement Feature’. • In 1992, first major revision to ISO standard occurred, referred to as SQL2 or SQL/92. • In 1999, SQL:1999 was released with support for object-oriented data management. • In late 2003, SQL:2003 was released.

  5. DBMS Languages • Data Definition Language (DDL) • Data Manipulation Language (DML) • High-Level or Non-procedural Languages: These include the relational language SQL • May be used in a standalone • or may be embedded in a programming language • Low Level or Procedural Languages: • These must be embedded in a programming language

  6. DBMS Languages • Data Definition Language (DDL): • Used by the DBA and database designers to specify the conceptual schema of a database. • In many DBMSs, the DDL is also used to define internal and external schemas (views). • In some DBMSs, separate storage definition language (SDL) and view definition language (VDL) are used to define internal and external schemas. • SDL is typically realized via DBMS commands provided to the DBA and database designers

  7. DBMS Languages • Data Manipulation Language (DML): • Used to specify database retrievals and updates • DML commands can be embedded in a general-purpose programming language (host language), such as C++, or Java. • A library of functions can also be provided to access the DBMS from a programming language • Alternatively, stand-alone DML commands can be applied directly (called a query language).

  8. Types of DML • High Level or Non-procedural Language: • For example, the SQL relational language • Are “set”-oriented and specify what data to retrieve rather than how to retrieve it. • Also called declarative languages. • Low Level or Procedural Language: • Retrieve data one record-at-a-time; • Constructs such as looping are needed to retrieve multiple records, along with positioning pointers.

  9. Objectives of SQL • Ideally, database language should allow user to: • create the database and relation structures; • perform insertion, modification, deletion of data from relations; • perform simple and complex queries. • It must be portable. • SQL is relatively easy to learn: • it is non-procedural - you specify what information you require, rather than how to get it • Can be used by range of users including DBAs, management, application developers, and other types of end users.

  10. Objectives of SQL • Consists of standard English words: 1) CREATE TABLE Staff( staffNo VARCHAR(5), lName VARCHAR(15), salary INTEGER); 2) INSERT INTO Staff VALUES (‘SG16’, ‘Brown’, 8300); 3) SELECT staffNo, lName, salary FROM Staff WHERE salary > 10000;

  11. Writing SQL Commands • SQL statement consists of reserved words and user-defined words. • Most components of an SQL statement are case insensitive, except for literal character data.

  12. Writing SQL Commands - Upper-case letters represent reserved words. - Lower-case letters represent user-defined words. - | indicates a choice among alternatives. - Curly braces indicate a required element. - Square brackets indicate an optional element. - … indicates optional repetition (0 or more).

  13. Literals • Literals are constants used in SQL statements. • All non-numeric literals must be enclosed in single quotes (e.g. ‘London’). • All numeric literals must not be enclosed in quotes (e.g. 650.00).

  14. Attribute Data Types in SQL • Basic data types • Numeric data types • Integer numbers: INTEGERand SMALLINT • Floating-point (real) numbers: FLOAT or REAL, and DOUBLE PRECISION • Character-string data types • Fixed length: CHAR(n) • Varying length: VARCHAR(n)

  15. Attribute Data Types in SQL (cont’d.) • 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

  16. Attribute Data Types in SQL (cont’d.) • Additional data types • TIME: • Made up of hour:minute:second in the format hh:mm:ss • TIME(i): • Made up of hour:minute:second plus i additional digits specifying fractions of a second • format is hh:mm:ss:ii...i • 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

  17. Attribute Data Types in SQL (cont’d.) • Additional data types • INTERVAL data type • Specifies a relative value that can be used to increment or decrement an absolute value of a date, time, or timestamp • Can be DAY/TIME intervals or YEAR/MONTH intervals

  18. Data Definition, Constraints, and Schema Changes • Used to CREATE, DROP, and ALTER the descriptions of the tables (relations) of a database

  19. CREATE TABLE 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 ); • CREATE TABLE command can be used for specifying the primary key attributes, secondary key, and referential integrity constraints (foreign keys). • Key attributes can be specified via the PRIMARY KEY and UNIQUE phrases • Specifies a new base relation by giving it a name, and specifying each of its attributes and their data types (INTEGER, FLOAT, DECIMAL(i,j), CHAR(n), VARCHAR(n)) • A constraint NOT NULL may be specified on an attribute

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

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

  22. 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; • UNIQUEclause • Specifies alternate (secondary) keys • Dname VARCHAR(15) UNIQUE;

  23. Specifying Key and Referential Integrity Constraints (cont’d.) • FOREIGNKEYclause • Default operation: reject update on violation • Attach referential triggered action clause • Options include RESTRICT, SET NULL, CASCADE, and SET DEFAULT • CASCADE option suitable for “relationship” relations

  24. REFERENTIAL INTEGRITY OPTIONS CREATE TABLE DEPT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9) DEFAULT ‘NO’, MGRSTARTDATE CHAR(9), PRIMARY KEY (DNUMBER), UNIQUE (DNAME), FOREIGN KEY (MGRSSN) REFERENCES EMP ON DELETE SET DEFAULT ON UPDATE CASCADE);

  25. REFERENTIAL INTEGRITY OPTIONS (continued) CREATE TABLE EMP(ENAME VARCHAR(30) NOT NULL,ESSN CHAR(9),BDATE DATE,DNO INTEGER DEFAULT 1,SUPERSSN CHAR(9),PRIMARY KEY (ESSN),FOREIGN KEY (DNO) REFERENCES DEPT ON DELETE SET DEFAULT ON UPDATE CASCADE,FOREIGN KEY (SUPERSSN) REFERENCES EMP ON DELETE SET NULL ON UPDATE CASCADE);

  26. Giving Names to Constraints • Keyword CONSTRAINT • Name a constraint • Useful for later altering

  27. 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);

  28. The DROP Command • DROP command • Used to remove named schema elements, such as tables, domains, or constraint • Drop behavior options: • CASCADE and RESTRICT • Examples:DROP TABLE DEPENDENT; DROP SCHEMA COMPANY CASCADE;

  29. The ALTER Command • Alter table actions include: • Adding or dropping a column (attribute) • The new attribute will have NULLs in all the tuples of the relation right after the command is executed • the NOT NULL constraint is not allowed for such an attribute • Changing a column definition • Adding or dropping table constraints • Examples: • ALTER TABLE COMPANY.EMPLOYEE ADD COLUMN Job VARCHAR(12); • Or: ALTER TABLE EMPLOYEE ADD JOB VARCHAR(12);

  30. The ALTER Command (cont’d.) • 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. • To drop a column • Choose either CASCADE or RESTRICT • Change constraints specified on a table • Add or drop a named constraint

  31. ALTER TABLE Examples • ALTER TABLE EMPLOYEE ADD JOB VARCHAR(12); • ALTER TABLE EMPLOYEE DROP ADDRESS CASCADE; • ALTER TABLE DEPARTMENT ALTER MGRSSN DROP DEFAULT; • ALTER TABLE DEPARTMENT ALTER MGRSSN SET DEFAULT "333445555";

  32. Retrieval Queries in SQL • SQL has one basic statement for retrieving information from a database; the SELECT statement • This is not the same as the SELECT operation of the relational algebra • Important distinction between SQL and the formal relational model: • SQL allows a table (relation) to have two or more tuples that are identical in all their attribute values • SQL relations can be constrained to be sets by specifying PRIMARY KEY or UNIQUE attributes, or by using the DISTINCT option in a query

  33. Retrieval Queries in SQL (contd.) • Basic form of the SQL SELECT statement is called a mapping or a SELECT-FROM-WHERE block SELECT <attribute list> FROM <table list> WHERE <condition> • <attribute list> is a list of attribute names whose values are to be retrieved by the query • <table list> is a list of the relation names required to process the query • <condition> is a conditional (Boolean) expression that identifies the tuples to be retrieved by the query • Logical comparison operators: =, <, <=, >, >=, and <>

  34. Relational Database Schema--Figure 5.5

  35. Populated Database--Fig.5.6

  36. Simple SQL Queries • Basic SQL queries correspond to using the following operations of the relational algebra: • SELECT • PROJECT • JOIN • All subsequent examples use the COMPANY database

  37. Simple SQL Queries (contd.) • Example of a simple query on one relation • Query 0: Retrieve the birthdate and address of the employee whose name is 'John B. Smith'. Q0:SELECT BDATE, ADDRESS FROM EMPLOYEE WHERE FNAME='John' AND MINIT='B’ AND LNAME='Smith’ • Similar to a SELECT-PROJECT pair of relational algebra operations: • The SELECT-clause specifies the projection attributes and the WHERE-clause specifies the selection condition • However, the result of the query may contain duplicate tuples

  38. Simple SQL Queries (contd.) • Query 1: Retrieve the name and address of all employees who work for the 'Research' department. Q1: SELECT FNAME, LNAME, ADDRESS FROM EMPLOYEE, DEPARTMENT WHERE DNAME='Research' AND DNUMBER=DNO • Similar to a SELECT-PROJECT-JOIN sequence of relational algebra operations • (DNAME='Research') is a selection condition (corresponds to a SELECT operation in relational algebra) • (DNUMBER=DNO) is a join condition (corresponds to a JOIN operation in relational algebra)

  39. Simple SQL Queries (contd.) • Query 2: For every project located in 'Stafford', list the project number, the controlling department number, and the department manager's last name, address, and birthdate. Q2: SELECT PNUMBER, DNUM, LNAME, BDATE, ADDRESS FROM PROJECT, DEPARTMENT, EMPLOYEE WHERE DNUM=DNUMBER AND MGRSSN=SSN AND PLOCATION='Stafford' • In Q2, there are two join conditions • The join condition DNUM=DNUMBER relates a project to its controlling department • The join condition MGRSSN=SSN relates the controlling department to the employee who manages that department

  40. Ambiguous Attribute Names • In SQL, we can use the same name for two (or more) attributes as long as the attributes are in different relations • A query that refers to two or more attributes with the same name must qualify the attribute name with the relation name by prefixing the relation name to the attribute name • Example: EMPLOYEE.LNAME, DEPARTMENT.DNAME

  41. ALIASES • Declare alternative relation names • Some queries need to refer to the same relation twice • In this case, aliases are given to the relation name • Query 8: For each employee, retrieve the employee's name, and the name of his or her immediate supervisor.Q8: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE ES WHERE E.SUPERSSN=S.SSN • In Q8, the alternate relation names E and S are called aliases or tuple variables for the EMPLOYEE relation • We can think of E and S as two different copies of EMPLOYEE; E represents employees in role of supervisees and S represents employees in role of supervisors

  42. ALIASES (contd.) • Aliasing can also be used in any SQL query for convenience • Can also use the AS keyword to specify aliases Q8: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE AS E, EMPLOYEE AS S WHERE E.SUPERSSN=S.SSN • Renaming of Attributes • Rename any attribute that appears in the result of a query

  43. UNSPECIFIED WHERE-clause • A missing WHERE-clause indicates no condition; • All tuples of the relations in the FROM-clause are selected • This is equivalent to the condition WHERE TRUE • Query 9: Retrieve the SSN values for all employees. • Q9: SELECT SSN FROM EMPLOYEE • If more than one relation is specified in the FROM-clause and there is no join condition, then the CARTESIAN PRODUCT of tuples is selected

  44. UNSPECIFIED WHERE-clause (contd.) • Example: Q10: SELECT SSN, DNAME FROM EMPLOYEE, DEPARTMENT • It is extremely important not to overlook specifying any selection and join conditions in the WHERE-clause; otherwise, incorrect and very large relations may result

  45. USE OF ASTERISK * • To retrieve all the attribute values of the selected tuples, a * is used, which stands for all the attributes Examples: Q1C: SELECT * FROM EMPLOYEE WHERE DNO=5Q1D: SELECT * FROM EMPLOYEE, DEPARTMENT WHERE DNAME='Research' AND DNO=DNUMBER

  46. USE OF DISTINCT • SQL does not treat a relation as a set; duplicate tuples can appear • To eliminate duplicate tuples in a query result, the keyword DISTINCT is used • For example, the result of Q11 may have duplicate SALARY values whereas Q11A does not have any duplicate values Q11: SELECT SALARY FROM EMPLOYEEQ11A: SELECT DISTINCT SALARY FROM EMPLOYEE

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