Chapter08

Chapter 8 SQL-99: SchemaDefinition, Constraints, and Queries and Views

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

CREATE TABLE 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 CREATE TABLE DEPARTMENT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9), MGRSTARTDATE CHAR(9) );

CREATE TABLE In SQL2, can use the CREATE TABLE command for specifying the primary key attributes, secondary keys, and referential integrity constraints (foreign keys). Key attributes can be specified via the PRIMARY KEY and UNIQUE phrases 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 );

DROP TABLE Used to remove a relation (base table) and its definition The relation can no longer be used in queries, updates, or any other commands since its description no longer exists Example: DROP TABLE DEPENDENT;

ALTER TABLE Used to add an attribute to one of the base relations The new attribute will have NULLs in all the tuples of the relation right after the command is executed; hence, 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.

REFERENTIAL INTEGRITY OPTIONS We can specify RESTRICT, CASCADE, SET NULL or SET DEFAULT on referential integrity constraints (foreign keys) 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 ON DELETE SET DEFAULT ON UPDATE CASCADE);

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);

Additional Data Types in SQL2 and SQL-99 Has DATE, TIME, and TIMESTAMP data types DATE: Made up of year-month-day in the format yyyy-mm-dd 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

Additional Data Types in SQL2 and SQL-99 (contd.) TIMESTAMP: Has both DATE and TIME components INTERVAL: Specifies a relative value rather than an absolute value Can be DAY/TIME intervals or YEAR/MONTH intervals Can be positive or negative when added to or subtracted from an absolute value, the result is an absolute value

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 Hence, an SQL relation (table) is a multi-set (sometimes called a bag ) of tuples; it is not a set of tuples SQL relations can be constrained to be sets by specifying PRIMARY KEY or UNIQUE attributes, or by using the DISTINCT option in a query

Retrieval Queries in SQL (contd.) A bag or multi-set is like a set, but an element may appear more than once. Example: {A, B, C, A} is a bag. {A, B, C} is also a bag that also is a set. Bags also resemble lists, but the order is irrelevant in a bag. Example: {A, B, A} = {B, A, A} as bags However, [A, B, A] is not equal to [B, A, A] as lists

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

Relational Database Schema--Figure 5.5

Simple SQL Queries 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’ 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

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

Aliases, * and DISTINCT, Empty WHERE-clause 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

ALIASES 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 E S 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

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

UNSPECIFIED WHERE-clause A missing WHERE-clause indicates no condition; hence, 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

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

USE OF * 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=5 Q1D: SELECT * FROM EMPLOYEE, DEPARTMENT WHERE DNAME='Research' AND DNO=DNUMBER

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 EMPLOYEE Q11A: SELECT DISTINCT SALARY FROM EMPLOYEE

SET OPERATIONS SQL has directly incorporated some set operations There is a union operation (UNION), and in some versions of SQL there are set difference (MINUS) and intersection (INTERSECT) operations The resulting relations of these set operations are sets of tuples; duplicate tuples are eliminated from the result The set operations apply only to union compatible relations ; the two relations must have the same attributes and the attributes must appear in the same order

SET OPERATIONS (contd.) Query 4: Make a list of all project numbers for projects that involve an employee whose last name is 'Smith' as a worker or as a manager of the department that controls the project. Q4: (SELECT PNAME FROM PROJECT, DEPARTMENT, EMPLOYEE WHERE DNUM=DNUMBER AND MGRSSN=SSN AND LNAME='Smith') UNION (SELECT PNAME FROM PROJECT, WORKS_ON, EMPLOYEE WHERE PNUMBER=PNO AND ESSN=SSN AND NAME='Smith')

NESTING OF QUERIES A complete SELECT query, called a nested query , can be specified within the WHERE-clause of another query, called the outer query Many of the previous queries can be specified in an alternative form using nesting Query 1: Retrieve the name and address of all employees who work for the 'Research' department. Q1: SELECT FNAME, LNAME, ADDRESS FROM EMPLOYEE WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research' )

NESTING OF QUERIES (contd.) The nested query selects the number of the 'Research' department The outer query select an EMPLOYEE tuple if its DNO value is in the result of either nested query The comparison operator IN compares a value v with a set (or multi-set) of values V, and evaluates to TRUE if v is one of the elements in V In general, we can have several levels of nested queries A reference to an unqualified attribute refers to the relation declared in the innermost nested query In this example, the nested query is not correlated with the outer query

CORRELATED NESTED QUERIES If a condition in the WHERE-clause of a nested query references an attribute of a relation declared in the outer query , the two queries are said to be correlated The result of a correlated nested query is different for each tuple (or combination of tuples) of the relation(s) the outer query Query 12: Retrieve the name of each employee who has a dependent with the same first name as the employee. Q12: SELECT E.FNAME, E.LNAME FROM EMPLOYEE AS E WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT WHERE ESSN=E.SSN AND E.FNAME=DEPENDENT_NAME)

CORRELATED NESTED QUERIES (contd.) In Q12, the nested query has a different result in the outer query A query written with nested SELECT... FROM... WHERE... blocks and using the = or IN comparison operators can always be expressed as a single block query. For example, Q12 may be written as in Q12A Q12A: SELECT E.FNAME, E.LNAME FROM EMPLOYEE E, DEPENDENT D WHERE E.SSN=D.ESSN AND E.FNAME=D.DEPENDENT_NAME

CORRELATED NESTED QUERIES (contd.) The original SQL as specified for SYSTEM R also had a CONTAINS comparison operator, which is used in conjunction with nested correlated queries This operator was dropped from the language , possibly because of the difficulty in implementing it efficiently Most implementations of SQL do not have this operator The CONTAINS operator compares two sets of values , and returns TRUE if one set contains all values in the other set Reminiscent of the division operation of algebra

CORRELATED NESTED QUERIES (contd.) Query 3: Retrieve the name of each employee who works on all the projects controlled by department number 5. Q3: SELECT FNAME, LNAME FROM EMPLOYEE WHERE ( (SELECT PNO FROM WORKS_ON WHERE SSN=ESSN) CONTAINS (SELECT PNUMBER FROM PROJECT WHERE DNUM=5) )

CORRELATED NESTED QUERIES (contd.) In Q3, the second nested query, which is not correlated with the outer query, retrieves the project numbers of all projects controlled by department 5 The first nested query, which is correlated, retrieves the project numbers on which the employee works, which is different for each employee tuple because of the correlation

THE EXISTS FUNCTION EXISTS is used to check whether the result of a correlated nested query is empty (contains no tuples) or not We can formulate Query 12 in an alternative form that uses EXISTS as Q12B

THE EXISTS FUNCTION (contd.) Query 12: Retrieve the name of each employee who has a dependent with the same first name as the employee. Q12B: SELECT FNAME, LNAME FROM EMPLOYEE WHERE EXISTS (SELECT * FROM DEPENDENT WHERE SSN=ESSN AND FNAME=DEPENDENT_NAME)

THE EXISTS FUNCTION (contd.) Query 6: Retrieve the names of employees who have no dependents. Q6: SELECT FNAME, LNAME FROM EMPLOYEE WHERE NOT EXISTS (SELECT * FROM DEPENDENT WHERE SSN=ESSN) In Q6, the correlated nested query retrieves all DEPENDENT tuples related to an EMPLOYEE tuple. If none exist , the EMPLOYEE tuple is selected

EXPLICIT SETS It is also possible to use an explicit (enumerated) set of values in the WHERE-clause rather than a nested query Query 13: Retrieve the social security numbers of all employees who work on project number 1, 2, or 3. Q13: SELECT DISTINCT ESSN FROM WORKS_ON WHERE PNO IN (1, 2, 3)

NULLS IN SQL QUERIES SQL allows queries that check if a value is NULL (missing or undefined or not applicable) SQL uses IS or IS NOT to compare NULLs because it considers each NULL value distinct from other NULL values, so equality comparison is not appropriate . Query 14: Retrieve the names of all employees who do not have supervisors. Q14: SELECT FNAME, LNAME FROM EMPLOYEE WHERE SUPERSSN IS NULL Note: If a join condition is specified, tuples with NULL values for the join attributes are not included in the result

Joined Relations Feature in SQL2 Can specify a "joined relation" in the FROM-clause Looks like any other relation but is the result of a join Allows the user to specify different types of joins (regular "theta" JOIN, NATURAL JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, CROSS JOIN, etc)

Joined Relations Feature in SQL2 (contd.) Examples: Q8: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE E S WHERE E.SUPERSSN=S.SSN can be written as: Q8: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME FROM (EMPLOYEE E LEFT OUTER JOIN EMPLOYEE S ON E.SUPERSSN=S.SSN)

Joined Relations Feature in SQL2 (contd.) Examples: Q1: SELECT FNAME, LNAME, ADDRESS FROM EMPLOYEE, DEPARTMENT WHERE DNAME='Research' AND DNUMBER=DNO could be written as: Q1: SELECT FNAME, LNAME, ADDRESS FROM (EMPLOYEE JOIN DEPARTMENT ON DNUMBER=DNO) WHERE DNAME='Research’ or as: Q1: SELECT FNAME, LNAME, ADDRESS FROM (EMPLOYEE NATURAL JOIN DEPARTMENT AS DEPT(DNAME, DNO, MSSN, MSDATE) WHERE DNAME='Research’

Joined Relations Feature in SQL2 (contd.) Another Example: Q2 could be written as follows; this illustrates multiple joins in the joined tables Q2: SELECT PNUMBER, DNUM, LNAME, BDATE, ADDRESS FROM (PROJECT JOIN DEPARTMENT ON DNUM=DNUMBER) JOIN EMPLOYEE ON MGRSSN=SSN) ) WHERE PLOCATION='Stafford’

AGGREGATE FUNCTIONS Include COUNT, SUM, MAX, MIN, and AVG Query 15: Find the maximum salary, the minimum salary, and the average salary among all employees. Q15: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY) FROM EMPLOYEE

AGGREGATE FUNCTIONS (contd.) Query 16: Find the maximum salary, the minimum salary, and the average salary among employees who work for the 'Research' department. Q16: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY) FROM EMPLOYEE, DEPARTMENT WHERE DNO=DNUMBER AND DNAME='Research'

AGGREGATE FUNCTIONS (contd.) Queries 17 and 18: Retrieve the total number of employees in the company (Q17), and the number of employees in the 'Research' department (Q18). Q17: SELECT COUNT (*) FROM EMPLOYEE Q18: SELECT COUNT (*) FROM EMPLOYEE, DEPARTMENT WHERE DNO=DNUMBER AND DNAME='Research’

GROUPING In many cases, we want to apply the aggregate functions to subgroups of tuples in a relation Each subgroup of tuples consists of the set of tuples that have the same value for the grouping attribute(s) The function is applied to each subgroup independently SQL has a GROUP BY -clause for specifying the grouping attributes, which must also appear in the SELECT-clause

GROUPING (contd.) Query 20: For each department, retrieve the department number, the number of employees in the department, and their average salary. Q20: SELECT DNO , COUNT (*), AVG (SALARY) FROM EMPLOYEE GROUP BY DNO In Q20, the EMPLOYEE tuples are divided into groups- Each group having the same value for the grouping attribute DNO The COUNT and AVG functions are applied to each such group of tuples separately The SELECT-clause includes only the grouping attribute and the functions to be applied on each group of tuples A join condition can be used in conjunction with grouping

GROUPING (contd.) Query 21: For each project, retrieve the project number, project name, and the number of employees who work on that project. Q21: SELECT PNUMBER, PNAME, COUNT (*) FROM PROJECT, WORKS_ON WHERE PNUMBER=PNO GROUP BY PNUMBER, PNAME In this case, the grouping and functions are applied after the joining of the two relations

THE HAVING-CLAUSE Sometimes we want to retrieve the values of these functions for only those groups that satisfy certain conditions The HAVING -clause is used for specifying a selection condition on groups (rather than on individual tuples)

THE HAVING-CLAUSE (contd.) Query 22: For each project on which more than two employees work , retrieve the project number, project name, and the number of employees who work on that project. Q22: SELECT PNUMBER, PNAME, COUNT(*) FROM PROJECT, WORKS_ON WHERE PNUMBER=PNO GROUP BY PNUMBER, PNAME HAVING COUNT (*) > 2

SUBSTRING COMPARISON The LIKE comparison operator is used to compare partial strings Two reserved characters are used: ' % ' (or ' * ' in some implementations) replaces an arbitrary number of characters, and ' _ ' replaces a single arbitrary character

SUBSTRING COMPARISON (contd.) Query 25: Retrieve all employees whose address is in Houston, Texas. Here, the value of the ADDRESS attribute must contain the substring 'Houston,TX‘ in it. Q25: SELECT FNAME, LNAME FROM EMPLOYEE WHERE ADDRESS LIKE '%Houston,TX%'

SUBSTRING COMPARISON (contd.) Query 26: Retrieve all employees who were born during the 1950s. Here, '5' must be the 8th character of the string (according to our format for date), so the BDATE value is '_______5_', with each underscore as a place holder for a single arbitrary character. Q26: SELECT FNAME, LNAME FROM EMPLOYEE WHERE BDATE LIKE '_______5_’

ARITHMETIC OPERATIONS The standard arithmetic operators '+', '-'. '*', and '/' (for addition, subtraction, multiplication, and division, respectively) can be applied to numeric values in an SQL query result Query 27: Show the effect of giving all employees who work on the 'ProductX' project a 10% raise. Q27: SELECT FNAME, LNAME, 1.1*SALARY FROM EMPLOYEE, WORKS_ON, PROJECT WHERE SSN=ESSN AND PNO=PNUMBER AND PNAME='ProductX’

ORDER BY The ORDER BY clause is used to sort the tuples in a query result based on the values of some attribute(s) Query 28: Retrieve a list of employees and the projects each works in, ordered by the employee's department, and within each department ordered alphabetically by employee last name. Q28: SELECT DNAME, LNAME, FNAME, PNAME FROM DEPARTMENT, EMPLOYEE, WORKS_ON, PROJECT WHERE DNUMBER=DNO AND SSN=ESSN AND PNO=PNUMBER ORDER BY DNAME, LNAME

ORDER BY (contd.) The default order is in ascending order of values We can specify the keyword DESC if we want a descending order; the keyword ASC can be used to explicitly specify ascending order, even though it is the default

Summary of SQL Queries A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory. The clauses are specified in the following order: SELECT <attribute list> FROM <table list> [ WHERE <condition>] [ GROUP BY <grouping attribute(s)>] [ HAVING <group condition>] [ ORDER BY <attribute list>]

Summary of SQL Queries (contd.) The SELECT-clause lists the attributes or functions to be retrieved The FROM-clause specifies all relations (or aliases) needed in the query but not those needed in nested queries The WHERE-clause specifies the conditions for selection and join of tuples from the relations specified in the FROM-clause GROUP BY specifies grouping attributes HAVING specifies a condition for selection of groups ORDER BY specifies an order for displaying the result of a query A query is evaluated by first applying the WHERE-clause, then GROUP BY and HAVING, and finally the SELECT-clause

Specifying Updates in SQL There are three SQL commands to modify the database: INSERT , DELETE , and UPDATE

INSERT In its simplest form, it is used to add one or more tuples to a relation Attribute values should be listed in the same order as the attributes were specified in the CREATE TABLE command

INSERT (contd.) Example: U1: INSERT INTO EMPLOYEE VALUES ('Richard','K','Marini', '653298653', '30-DEC-52', '98 Oak Forest,Katy,TX', 'M', 37000,'987654321', 4 ) An alternate form of INSERT specifies explicitly the attribute names that correspond to the values in the new tuple Attributes with NULL values can be left out Example: Insert a tuple for a new EMPLOYEE for whom we only know the FNAME, LNAME, and SSN attributes. U1A: INSERT INTO EMPLOYEE (FNAME, LNAME, SSN) VALUES ('Richard', 'Marini', '653298653')

INSERT (contd.) Important Note: Only the constraints specified in the DDL commands are automatically enforced by the DBMS when updates are applied to the database Another variation of INSERT allows insertion of multiple tuples resulting from a query into a relation

INSERT (contd.) Example: Suppose we want to create a temporary table that has the name, number of employees, and total salaries for each department. A table DEPTS_INFO is created by U3A, and is loaded with the summary information retrieved from the database by the query in U3B. U3A: CREATE TABLE DEPTS_INFO (DEPT_NAME VARCHAR(10), NO_OF_EMPS INTEGER, TOTAL_SAL INTEGER); U3B: INSERT INTO DEPTS_INFO (DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) SELECT DNAME, COUNT (*), SUM (SALARY) FROM DEPARTMENT, EMPLOYEE WHERE DNUMBER=DNO GROUP BY DNAME ;

INSERT (contd.) Note: The DEPTS_INFO table may not be up-to-date if we change the tuples in either the DEPARTMENT or the EMPLOYEE relations after issuing U3B. We have to create a view (see later) to keep such a table up to date.

DELETE Removes tuples from a relation Includes a WHERE-clause to select the tuples to be deleted Referential integrity should be enforced Tuples are deleted from only one table at a time (unless CASCADE is specified on a referential integrity constraint) A missing WHERE-clause specifies that all tuples in the relation are to be deleted; the table then becomes an empty table The number of tuples deleted depends on the number of tuples in the relation that satisfy the WHERE-clause

DELETE (contd.) Examples: U4A: DELETE FROM EMPLOYEE WHERE LNAME='Brown’ U4B: DELETE FROM EMPLOYEE WHERE SSN='123456789’ U4C: DELETE FROM EMPLOYEE WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research') U4D: DELETE FROM EMPLOYEE

UPDATE Used to modify attribute values of one or more selected tuples A WHERE-clause selects the tuples to be modified An additional SET-clause specifies the attributes to be modified and their new values Each command modifies tuples in the same relation Referential integrity should be enforced

UPDATE (contd.) Example: Change the location and controlling department number of project number 10 to 'Bellaire' and 5, respectively. U5: UPDATE PROJECT SET PLOCATION = 'Bellaire', DNUM = 5 WHERE PNUMBER=10

UPDATE (contd.) Example: Give all employees in the 'Research' department a 10% raise in salary. U6: UPDATE EMPLOYEE SET SALARY = SALARY *1.1 WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research') In this request, the modified SALARY value depends on the original SALARY value in each tuple The reference to the SALARY attribute on the right of = refers to the old SALARY value before modification The reference to the SALARY attribute on the left of = refers to the new SALARY value after modification

Recap of SQL Queries A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory. The clauses are specified in the following order: SELECT <attribute list> FROM <table list> [ WHERE <condition>] [ GROUP BY <grouping attribute(s)>] [ HAVING <group condition>] [ ORDER BY <attribute list>] There are three SQL commands to modify the database: INSERT , DELETE , and UPDATE

Views (Virtual Tables) in SQL A view is a single table that is derived from other tables. These other tables can be base tables or previously defined views. A view does not necessarily exist in physical form; it is considered a virtual table It is a way of specifying a table that we need to reference frequently.

View Creation V1: CREATE VIEW WORKS_ON1 AS SELECT Fname, Lname, Pname, Hours FROM EPLOYEE, PROJECT, WORKS_ON WHERE Ssn=Essn and Pno=Pnumber V2: CREATE VIEW DEPT_INFO(Dname, EmpNo, TotalSalary) AS SELECT Dname, COUNT(*), SUM(Salary) FROM DEPARTMENT, EMPLOYEE WHERE Dnumber=Dno GROUP BY Dname

View Usage QV1: SELECT Fname, Lname FROM WORKS_ON1 WHERE Pname=“ProjectX”; View are always up-to-date because they are realized at the time of view execution rather than at the time of view definition.

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