1. CS 380 Introduction to Database Systems (Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views)

2. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views1 Outline Introduction Data Definition Language CREATE TABLE ALTER TABLE DROP TABLE Data Manipulation Language SELECT Statement INSERT Statement DELETE Statement UPDATE Statement Assertions Views

3. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views2 Introduction SQL: Structured Query Language. Standard for commercial relational DBMSs, a joint effort by ANSI & ISO. Benefits of a standardized relational language: Reduced training costs. Productivity. Application portability. Application longevity. Reduced dependence on a single vendor. Cross-system communication.

4. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views3 “Declarative” Versus “Procedural” Most DBMSs provide the user with a high-level declarative language interface. The user has just to specify “what” results he/she would like, leaving the “how” to the DBMS.

5. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views4 Schema and Catalog Concepts in SQL The concept of an SQL Schema was incorporated to group together tables & other constructs that belong to the same database application. Schema: the structure that contains descriptions of objects created by the user (tables, constraints, views, domains, character sets, triggers, rules). Catalog: a set of schemas that constitute the description of a database.

6. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views5 Types of SQL Statements Data Definition Language (DDL): Commands that define a database. E.g. CREATE, ALTER, DROP,...etc. Data manipulation language (DML): Commands that maintain and query a database. E.g. SELECT, INSERT, UPDATE, DELETE. Data Control Language (DCL): Commands that control a database, including administering privileges and committing data. E.g. CONNECT, GRANT, REVOKE,...etc.

7. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views6 CREATE Statements Major CREATE statements: CREATE SCHEMA: Defines a portion of the database owned by a particular user. CREATE SCHEMA COMPANY AUTHORIZATION JSMITH; CREATE TABLE: Defines a table and its columns. CREATE VIEW: Defines a logical table from one or more views. Other CREATE statements: DOMAIN, CHARACTER SET, COLLATION, TRANSLATION, ASSERTION.

8. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views7 CREATE TABLE Construct CREATE TABLE [schema.] name (column-1 TYPE [DEFAULT value] [constraints], column-2 TYPE [DEFAULT value] [constraints], column-n TYPE [DEFAULT value] [constraints], [table-constraints]); Where: [schema.]: schema name followed by a dot. name: table name. column-1 to column-n: column names. TYPE: data type. [DEFAULT value]: optional default value. [constraints]: optional constraints, can be specified at column level or at table level.

9. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views8 Some SQL Data Types (From Oracle) CHAR(n): a fixed-length string n characters long (default 1, max 255 character). VARCHAR(n): a variable string up to n character long ( max 2000 ). INT: an integer number. DECIMAL(n,d): a number n digits long including d decimal places. DATE: a valid date YYYY-MM-DD.

10. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views9 CREATE DOMAIN Construct A domain can be declared, and the domain name is used with the attribute specification. This makes it easier to change the data type for a domain that is used by numerous attributes in a schema, and improves schema readability. Examples: CREATE DOMAIN SSN_TYPE AS CHAR(9); CREATE DOMAIN D_NUM AS INT CHECK (D_NUM>0 AND D_NUM<21);

11. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Constraints There are essentially five types of constraints: NOT NULL constraints. UNIQUE value constraints. PRIMARY KEY constraints. FOREIGN KEY constraints. CHECK constraints. The syntax of these constraints varies according to whether they are specified at column level or at table level. Any constraints which apply to multiple columns must be defined as table level constraints. 10

12. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Constraints The syntax for each of the column level constraints is: NOT NULL UNIQUE PRIMARY KEY REFERENCES table (key) CHECK (condition) 11

13. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Constraints The syntax for each of the table level constraints is: UNIQUE (column-list) [CONSTRAINT name] PRIMARY KEY (column-list) [CONSTRAINT name] FOREIGN KEY (column-list) REFERENCES table (column-list) CHECK (condition) An additional ON DELETE clause may be applied to a foreign key constraint: ON DELETE CASCADE causes all referencing child records to be deleted whenever a parent record is deleted. ON DELETE SET NULL causes all referencing child records to be set to NULL whenever a parent record is deleted. 12

14. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Steps in Table Creation Identify data types for attributes. Determine default values. Identify columns that can and cannot be null. Identify columns that must be unique (candidate keys). Identify primary key - foreign key mates. Identify constraints on columns (domain specifications). Create the table and associated indexes. 13 To improve access speed

15. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views CREATE TABLE Example CREATE TABLE EMPLOYEE (Fname VARCHAR(15) NOT NULL, Minit CHAR, Lname VARCHAR(15) NOT NULL, Ssn CHAR(9)NOT NULL, Bdate DATE, Address VARCHAR(30), Sex CHAR CHECK (LOWER(SEX) IN ('m', 'f')), Salary DECIMAL(10,2), Super_ssn CHAR(9), Dno INT NOT NULL, CONSTRAINT EMPPK PRIMARY KEY (Ssn), CONSTRAINT EMPSUPERFK FOREIGN KEY (Super_ssn) REFERENCES EMPLOYEE (Ssn) ON DELETE SET NULL, CONSTRAINT EMPDEPTFK FOREIGN KEY (Dno) REFERENCES DEPARTMENT (Dnumber) ON DELETE SET NULL); 14

16. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views ALTER TABLE Construct Tables can be changed in a number of ways using the SQL statement ALTER TABLE. Possible actions include: Adding or dropping a column. Adding or dropping a constraint. Changing a column definition. 15

17. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views ALTER TABLE Construct To add a column to the EMPLOYEE table: ALTER TABLE EMPLOYEE ADD Job VARCHAR(12); To add a constraint to the DEPARTMENT table: ALTER TABLE DEPARTMENT ADD CONSTRAINT DEPTMGRFK FOREIGN KEY (Mgr_ssn) REFERENCES EMPLOYEE (Ssn) ON DELETE SET NULL; 16

18. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views ALTER TABLE Construct To drop the attribute Address from the EMPLOYEE table: ALTER TABLE EMPLOYEE DROP COLUMN Address CASCADE CONSTRAINTS; To drop a constraint named EMPSUPERFK from EMPLOYEE table: ALTER TABLE EMPLOYEE DROP CONSTRAINT EMPSUPERFK CASCADE; 17

19. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views ALTER TABLE Construct To increase the width of Lname column: ALTER TABLE EMPLOYEE MODIFY Lname VARCHAR2(20); To add a NOT NULL constraint to the Address column: ALTER TABLE EMPLOYEE MODIFY Address NOT NULL; To drop an existing default clause: ALTER TABLE DEPARTMENT ALTER Mgr_ssn DROP DEFAULT; To define a new default clause: ALTER TABLE DEPARTMENT ALTER Mgr_ssn SET DEFAULT '333445555'; 18

20. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views DROP SCHEMA Construct The DROP SCHEMA command can be used to drop a schema. Syntax: DROP SCHEMA name [CASCADE or RESTRICT] Where: name: is the name of the schema. CASCADE: removes the schema and all its tables, domains, and other elements. RESTRICT: removes the schema only if it has no elements. 19

21. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views DROP TABLE Construct The DROP TABLE command deletes all information about the dropped relation from the database. Syntax: DROP TABLE name [CASCADE CONSTRAINTS] Where: name: is the name of the table to be dropped. CASCADE CONSTRAINTS: drops all referential integrity constraints which refer to keys in the dropped table. 20

22. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Basic Queries in SQL SQL has one basic statement for retrieving data from a database, the SELECT statement. This is not the same as the SELECT operation of the relational algebra. 21

23. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The SQL SELECT Statement It consists of the following basic clauses: SELECT specifies the columns which are to be retrieved FROM specifies the tables from which data is to be retrieved WHERE defines any selection criteria required GROUP BY specifies how output is to be grouped & summarized HAVING determines which group summaries to include in the output ORDER BY defines how the output will be stored 22 Mandatory

24. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The SELECT-FROM-WHERE Structure The basic form of the SELECT statement is: SELECT[DISTINCT] FROM WHERE Where: item-list: a comma-separated list of items to be retrieved. (database attributes, functions, fixed text,...etc.) table-list: a comma-separated list of tables from which the attributes are to be retrieved. condition: a conditional (Boolean) expression that identifies the tuples to be retrieved by the query. [DISTINCT]: ensures only unique tuples will be returned. The result of an SQL query is a relation (can be a multiset) 23

25. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The SELECT-FROM-WHERE Structure Query: retrieve the birthdate and address of the employee(s) whose name is ‘John B. Smith’. SELECTBdate, Address FROMEMPLOYEE WHEREFname= ' John ' AND Minit= ' B ' AND Lname= ' Smith '; Is it similar to a tuple relational expression? Any differences? SELECT Statement Relational Algebra SELECT clause project operation FROM clause cartesian product operation WHERE clause select condition 24

26. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The SELECT-FROM-WHERE Structure Query: retrieve the name and address of all employees who work for the ‘Research’ department. Query: for every project located in ‘Stafford’, list the project number, the controlling department number, and the department manager’s last name, address, and birthdate. 25 SELECTFname, Lname, Address FROM EMPLOYEE, DEPARTMENT WHEREDname='Research' AND Dnumber=Dno; SELECTPnumber, Dnum, Lname, Address, Bdate FROM PROJECT, DEPARTMENT, EMPLOYEE WHEREDnum=Dnumber AND Mgr_ssn=ssn AND Plocation='Stafford';

27. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Ambiguous Attribute Names, Aliasing, and Tuple Variables In SQL if the same attribute name is used in more than one relation, the attribute names must be qualified. Assume that DNO attribute of the EMPLOYEE relation was called DNUMBER. Query: retrieve the name and address of all employees who work for the ‘Research’ department. SELECTFname, Lname, Address FROM EMPLOYEE, DEPARTMENT WHEREDname= ' Research ' AND EMPLOYEE.Dnumber=DEPARTMENT.Dnumber; 26

28. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Ambiguous Attribute Names, Aliasing, and Tuple Variables Ambiguity also arise in the case of queries that refer to the same relation twice, as in the following example. Query: for each employee, retrieve the employee’s first and last name and the first and last name of his or her immediate supervisor. SELECTE.Fname, E.Lname, S.Fname, S.Lname FROMEMPLOYEE E, EMPLOYEE S WHEREE.Super_ssn=S.Ssn; 27

29. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Unspecified WHERE Clause Query: select all EMPLOYEE SSNs. SELECTSsn FROMEMPLOYEE; Query: select all combinations of EMPLOYEE SSN and DEPARTMENT DNAME. 28 SELECTSsn, Dname FROMEMPLOYEE, DEPARTMENT;

30. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The Use of Asterisk Query: retrieve all the attribute values of any EMPLOYEE who work in DEPARTMENT 5. SELECT* FROMEMPLOYEE WHEREDno =5; Query: retrieve all the attributes of an EMPLOYEE and all the attributes of the DEPARTMENT in which he or she works for every employee of the ‘Research’ department. 29 SELECT* FROMEMPLOYEE, DEPARTMENT WHEREDname='Research' AND Dno=Dnumber;

31. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Tables as Sets in SQL Query: retrieve the salary of every employee. SELECTSalary FROMEMPLOYEE; Query: retrieve all distinct salary values. SELECTDISTINCTSalary FROMEMPLOYEE; 30 A set

32. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The UNION, INTERSECT, & MINUS SET OPERATORS The relations resulting from these set operations are sets of tuples; that is, duplicate tuples are eliminated from the result. Query: make a list of all project numbers for projects that involve an employee whose last name is ‘Smith’, either as a worker or as a manager of the department that controls the project. SELECTDISTINCTPnumber FROMPROJECT, DEPARTMENT, EMPLOYEE WHEREDnum=Dnumber AND Mgr_ssn=Ssn AND Lname= ' Smith ' UNION SELECTDISTINCTPnumber FROMPROJECT, WORKS_ON, EMPLOYEE WHEREPnumber=Pno AND Essn=Ssn AND Lname= ' Smith '; 31 Union compatible

33. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Conditional Operators Main conditional operators include: =, <>, >, =, <=. Other conditional operators: IN: tests if an item value appears in a specified list of values. BETWEEN: tests if an item value lies (inclusively) between two specified values. IS NULL: tests if an item has a Null value. Null is neither zero (in a numeric item), nor spaces (in a character item). LIKE: tests if an item value matches a string containing wildcard characters. The wildcard characters are: %: meaning zero or more occurrences of any character. _: meaning a single occurrence of any characters. The keyword NOT can be used in conjunction with all the above operators to revise the test. 32

34. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Conditional Operators Query: retrieve the names of all employees whose salary is not (20000, 30000, or 40000). Query: retrieve all employees in department 5 whose salary is between $30,000 and $40,000. 33 SELECTFname, Lname FROMEMPLOYEE WHERESalary NOT IN (20000, 30000, 40000); SELECT* FROMEMPLOYEE WHERE(Salary BETWEEN 30000 AND 40000) AND Dno = 5;

35. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Conditional Operators Query: retrieve the names of all employees who do not have supervisors. Query: retrieve all employees whose address is in Houston, Texas. 34 SELECTFname, Lname FROMEMPLOYEE WHERESuper_ssn IS NULL; SELECTFname, Lname FROMEMPLOYEE WHEREAddress LIKE '%Houston, TX%';

36. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Arithmetic Operations The standard arithmetic operations (+, -, *, /) can be applied to numeric values or attributes with numeric domains. Query: show the resulting salaries if every employee working on the ‘ProductX’ project is given 10 percent raise. SELECTFname, Lname, 1.1*Salary INCREASED_SAL FROMEMPLOYEE, WORKS_ON, PROJECT WHERESsn=Essn AND Pno=Pnumber AND Pname='ProductX'; 35

37. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Ordering of Query Results SQL allows the user to order the tuples in the result of a query by the values of one or more attributes, using the ORDER BY clause. Query: retrieve a list of employees and the projects they are working on, ordered by department and, within each department, ordered alphabetically by last name, first name. SELECTDname, Lname, Fname, Pname FROMDEPARTMENT, EMPLOYEE, WORKS_ON, PROJECT WHEREDnumber=Dno AND Ssn=Essn AND Pno=Pnumber ORDER BYDname, Lname, Fname; The default order is in ascending order. ORDER BY Dname DESC, Lname ASC, Fname ASC 36

38. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Nested Queries Some queries require that existing values in the database be fetched and then used in a comparison condition. Such queries can be conveniently formulated by using nested queries. Nested queries: complete select-from-where blocks usually within the WHERE clause of another query. That other query is called the outer query. 38

39. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Nested Queries Query: make a list of all project numbers for projects that involve an employee whose last name is ‘Smith’, either as a worker or as a manager of the department that controls the project. SELECTDISTINCTPnumber FROMPROJECT WHEREPnumber IN (SELECT Pnumber FROMPROJECT, EMPLOYEE, DEPARTMENT WHERE Dnum=Dnumber AND Mgr_ssn=Ssn AND Lname='Smith') OR Pnumber IN (SELECT Pno FROMWORKS_ON, EMPLOYEE WHEREEssn=Ssn AND Lname='Smith'); 39

40. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Nested Queries SQL allows the use of tuples of values in comparisons by placing them within parentheses. Example: SELECTDISTINCTEssn FROMWORKS_ON WHERE(Pno, Hours) IN (SELECT Pno, Hours FROMWORKS_ON WHEREEssn='123456789'); 40

41. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Nested Queries In addition to the IN operator, a number of other comparison operators can be used to compare a single value (attribute) to a set or multiset. The keywords ANY or ALL can be combined with the following operators: >, >=,. (= ANY is equivalent to IN) Query: list the names of employees whose salary is greater than the the salary of all the employees in department 5. 41 SELECTLname, Fname FROMEMPLOYEE WHERESalary > ALL (SELECTSalary FROMEMPLOYEE WHEREDno=5);

42. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views44 The EXISTS Function in SQL This function checks whether the result of a correlated nested query is empty (contains no tuples) or not. Query: retrieve the name of each employee who has a dependent with the same first name and same sex as the employee. SELECTE.Fname, E.Lname FROMEMPLOYEE E WHEREEXISTS (SELECT * FROMDEPENDENT WHEREE.Ssn=Essn AND E.Sex=Sex AND E.Fname=Dependent_name);

43. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views45 The EXISTS Function in SQL Query: retrieve the names of employees who have no dependents. SELECTFname, Lname FROMEMPLOYEE WHERENOT EXISTS (SELECT* FROMDEPENDENT WHERESsn=Essn);

44. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Aggregate Function in SQL A number of built-in functions exist: COUNT, SUM, MAX, MIN, and AVG. These functions can be used in the SELECT clause or in the HAVING clause. The functions SUM, MAX, MIN, and AVG are applied to a set or multiset of numeric values. The functions MAX and MIN can also be used with attributes that have nonnumeric domains if the domain values have a total ordering among one another. Null values are discarded when aggregate functions are applied to a particular attribute. 50

45. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Aggregate Function in SQL Query: find the sum of the salaries of all employees, the maximum salary, the minimum salary, and the average salary. Query: find the sum of the salaries of all employees of the ‘Research’ department, as well as the maximum salary, the minimum salary, and the average salary in this department. 51 SELECTSUM (Salary), MAX (Salary), MIN (Salary), AVG (Salary) FROMEMPLOYEE; SELECTSUM (Salary), MAX (Salary), MIN (Salary), AVG (Salary) FROMEMPLOYEE, DEPARTMENT WHEREDno=Dnumber AND Dname='Research';

46. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Aggregate Function in SQL Query: retrieve the total number of employees in the company. Query: retrieve the total number of employees in the ‘Research’ department. 52 SELECTCOUNT(*) FROMEMPLOYEE; SELECTCOUNT(*) FROMEMPLOYEE, DEPARTMENT WHEREDno=Dnumber AND Dname='Research';

47. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Aggregate Function in SQL Query: Count the number of distinct salary values in the database. Query: retrieve the names of all employees who have two or more dependents. 53 SELECTCOUNT(DISTINCT Salary) FROMEMPLOYEE; SELECTLname, Fname FROMEMPLOYEE WHERE(SELECTCOUNT(*) FROMDEPENDENT WHERESsn=Essn) >= 2;

48. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The GROUP BY Clause This clause enables aggregates to be accumulated and grouped on output. Query: for each department, retrieve the department number, the number of employees in the department, and their average salary. SELECTDNO, COUNT(*), AVG(Salary) FROMEMPLOYEE GROUP BYDno; If Nulls exist in the grouping attribute, then a separate group is created for all tuples with a NULL value in the grouping attribute. 54

49. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The HAVING Clause This clause specifies which groups are to be selected for output. Groups are only output if they match the condition(s) specified by the HAVING clause. Query: for each project on which more than two employees work, retrieve the project number, the project name, and the number of employees who work on the project. SELECTPnumber, Pname, COUNT(*) FROMPROJECT, WORKS_ON WHEREPnumber=Pno GROUP BYPnumber, Pname HAVINGCOUNT(*) >2; 56

50. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views INSERT, DELETE, and UPDATE Statements In SQL, three commands can be used to modify the database: INSERT. DELETE. UPDATE. 59

51. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The INSERT Statement Using the SQL INSERT statement, rows may be inserted into tables in two ways: As lists of values, one row at a time. From another table using a query. 60

52. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views INSERT Using a Value List Syntax: INSERT INTO table [(attribute-list)] VALUES (value-list) Where: table: the name of the table to update. attribute-list: a list of attributes in the specified table. value-list: the list of values to be inserted for the row. 61

53. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views INSERT Using a Value List Example: INSERT INTO PROJECT VALUES ('ProductZ', 1, NULL, 2) Example: INSERT INTO PROJECT (PNAME, PNUMBER, DNUM) VALUES ('ProductZ', 1, 2) Example: INSERT INTO PROJECT (PNAME, PNUMBER, DNUM) VALUES ('&PN', &P_NO, &D_NO) 62 Each time it is run, the user will be prompted for a new set of values

54. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views INSERT Using a Query Syntax: INSERT INTO table [(attribute-list)] SELECT statement There are no limitation concerning the SELECT statement. 63

55. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views INSERT Using a Query Example: CREATE TABLEDEPTS_INFO (DEPT_NAMEVARCHAR(15), NO_OF_EMPSINTEGER, TOTAL_SALINTEGER); INSERT INTODEPTS_INFO SELECTDNAME, COUNT(*), SUM (Salary) FROMDEPARTMENT, EMPLOYEE WHEREDnumber=Dno GROUP BYDname; 64

56. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The DELETE Statement Syntax: DELETEFROM table [WHERE conditions] Example: DELETEFROM EMPLOYEE WHERELname='Brown'; Example: DELETEFROM EMPLOYEE WHEREDno IN (SELECTDnumber FROMDEPARTMENT WHEREDname='Research'); Example: DELETEFROM EMPLOYEE; 65

57. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views The UPDATE Statement The UPDATE statement can be used: With individual values. In conjunction with a SELECT statement. 66

58. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views UPDATE Using Fixed Values Syntax: UPDATEtable SETattribute-1 = value-1, attribute-2 = value-2, : attribute-n = value-n [WHERE conditions] Example: UPDATEPROJECT SETPlocation = 'Bellaire' WHEREPnumber=10; Example: UPDATEEMPLOYEE SETSalary = Salary * 1.25; 67

59. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views UPDATE Using a Query Syntax: UPDATEtable SET(attribute-list) = (SELECT statement) [WHERE conditions] Example: UpdateEMPLOYEE SET(Salary) = ( SELECTSalary FROMEMPLOYEE WHERESsn='987654321') WHEREFname = 'John' AND Lname='Smith'; 68

60. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Concept of a View (Virtual Table) in SQL A view is a customized representation of one or more underlying tables or other views (or a mixture of both). A view does not necessarily exist in physical form. Allows full query operations. Allows limited update operations (since the table may not physically be stored). Convenience for expressing certain operations. 70

61. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Concept of a View (Virtual Table) in SQL Advantages of views: Simplify query commands. Provide data security. Enhance programming productivity. 71

62. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Specification of Views in SQL Example: CREATE VIEWWORKS_ON1AS SELECTFname, Lname, Pname, Hours FROMEMPLOYEE, PROJECT, WORKS_ON WHERESsn=Essn AND Pno=Pnumber; Example: CREATE VIEWDEPT_INFO(Dept_name, No_of_emps, Total_sal) AS SELECTDname, COUNT(*), SUM(Salary) FROMDEPARTMENT, EMPLOYEE WHEREDnumber=Dno GROUP BYDname; 72

63. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Specification of Views in SQL Query: retrieve the last name and first name of all employees who work on ‘ProjectX’. SELECTFname, Lname FROMWORKS_ON1 WHEREPname='ProjectX'; A view is supposed to be always up to date. When no longer needed, a view can be dropped: DROP VIEW name 73

64. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views Effective View Implementation Two main approaches have been suggested: Query modification: involves modifying the view query into a query on the underlying base tables. I.e, the previous query would be automatically modified to: SELECT Fname, Lname FROM EMPLOYEE, PROJECT, WORKS_ON WHERE Ssn=Essn AND Pno=Pnumber AND Pname='ProjectX'; View Materialization: involves physically creating a temporary view table (should be kept up to date). 74

65. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views View Update A view with a single defining table is updatable if the view attributes contain the primary key of the base relation, as well as all attributes with the NOT NULL constraint that do not have default values specified. Views defined on multiple tables using joins are generally not updatable. Views defined using grouping and aggregate functions are not updatable. 75

66. Chapter 8: SQL-99: Schema Definition, Constraints, Queries, and Views View Update In SQL, the clause WITH CHECK OPTION must be added at the end of the new definition if the view is to be updated. 76