SQL database management System a brief intro

SQL: SCHEMA DEFINITION, BASIC
CONSTRAINTS, AND QUERIES

SQL INTRODUCTION
 SQL stands for Structured Query Language
 Standard language for querying and manipulating data
 Data Definition Language (DDL)
 Create/alter/delete tables and their attributes
 Data Manipulation Language (DML)
 Query one or more tables
 Insert/delete/modify tuples in tables
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Many standards out there: ANSI SQL, SQL92 (a.k.a. SQL2), SQL99 (a.k.a. SQL3), …

TABLES IN SQL
DName Dnumber MgrSsn Mgrstartdate
Gizmo 19 324521 1992-08-11
Powergizmo 29 624545 1982-01-21
SingleTouch 1 986133 1912-04-02
MultiTouch 2 1123455 2002-08-28
Department
Attribute names
Table name
Tuples or rows

CREATE TABLE
 Creates a new relation(table) in the database
 Specifies relation’s attributes and their data types
 Specifies constraints such as NOT NULL , UNIQUE ,CHECK etc…
CREATE TABLE DEPARTMENT
(DNAME VARCHAR(10) NOT NULL ,
DNUMBER INTEGER CHECK(DNUMBER >0 AND DNUMBER
<25),
MGRSSN CHAR(9),
MGRSTARTDATE DATE
);
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CREATE SCHEMA
 Specifies a new database schema by giving it a name
 Example:
CREATE SCHEMA COMPANY AUTHORIZATION
username;
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Data Types
 Numeric
 integer
 Smallint
 Floating point
 Decimal(Precision, Scale)
 Numeric(Precision,Scale)
 Character String
 Char(n)
 Varchar(n)
 Nvarchar(n)
 Bit-String
 Bolean

ADDITIONAL 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
 TIMESTAMP:
 Has both DATE and TIME components
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CREATE DOMAIN
 We can declare a domain in SQL
 This makes it easier to change the data type of
numerous attributes in a schema
 Improves Schema readability
 Not available in many SQL implementation like (T-SQL)
CREATE DOMAIN SSN_TYPE AS CHAR(9);
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CONSTRAINTS IN SQL
 CREATE TABLE command allows us to specify the
primary key, secondary keys, and foreign keys.
 Key attributes can be specified via the PRIMARY KEY and
UNIQUE phrases
( DNAME VARCHAR(10) NOT NULL,
DNUMBER INTEGER NOT NULL,
MGRSSN CHAR(9) NULL,
MGRSTARTDATE CHAR(9),
PRIMARY KEY (DNUMBER),
UNIQUE (DNAME),
FOREIGN KEY (MGRSSN) REFERENCES EMPLOYEE
); Slide
8-9

REFERENTIAL INTEGRITY OPTIONS
 We can specify RESTRICT, CASCADE, SET NULL or SET
DEFAULT on foreign keys.
( DNAME VARCHAR(10) NOT NULL,
DNUMBER INTEGER NOT NULL,
MGRSSN CHAR(9) ,
MGRSTARTDATE CHAR(9),
PRIMARY KEY (DNUMBER),
UNIQUE (DNAME),
FOREIGN KEY (MGRSSN) REFERENCES EMPLOYEE
ON DELETE SET DEFAULT ON UPDATE CASCADE
);
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CASCADE : to delete a row with a key referenced by foreign keys in existing rows in
other tables, all rows that contain those foreign keys are also deleted.

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CREATE TABLE EMPLOYEE
( ENAME VARCHAR(30) NOT NULL,
ESSN CHAR(9),
BDATE DATE,
DNO INTEGER DEFAULT 1,
SUPERSSN CHAR(9),
PRIMARY KEY (ESSN),
FOREIGN KEY (DNO) REFERENCES DEPARTMENT
ON DELETE SET DEFAULT ON UPDATE CASCADE,
FOREIGN KEY (SUPERSSN) REFERENCES EMPLOYEE
ON DELETE SET NULL ON UPDATE CASCADE
);
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SQL CONSTRAINTS
 Assigning Names to Constraints
CONSTRAINT deptPK PRIMARY KEY(Dnumber)
CONSTRAINT deptSK UNIQUE(Dname)
 CHECK Constraint
CHECK (Dept_create_date <= Mgr_start_date)
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DROP COMMAND
 Drop Command is used to delete schema or named
schema elements such as table, domains, or constraints
 Example:
DROP TABLE DEPENDENT;
DROP TABLE EMPLOYEE CASCADE;
DROP SCHEMA COMPANY;
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In SQL-Server (T-SQL), DROP TABLE cannot be used to drop a table that is referenced by a
FOREIGN KEY. The referencing FOREIGN KEY or the referencing table must first be dropped.

ALTER COMMAND
 The definition of table or named schema elements can be
changed using ALTER command
 ALTER can be used to add an attribute to the relation
 Initially, the new attribute will have NULLs in all the tuples of the
relation
 NOT NULL constraint is not allowed for such an attribute
 Example :
ALTER TABLE EMPLOYEE ADD COLUMN JOB VARCHAR(12);
T-SQL syntax
ALTER TABLE EMPLOYEE ADD JOB VARCHAR(12);
 The database user have to enter a value for the new attribute JOB for
each EMPLOYEE tuple. Slide
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ALTER TABLE
 ALTER command can be use to add or drop constraints
 Example :
ALTER TABLE EMPLOYEE add constraint unEmp UNIQUE(NAME) ;
ALTER TABLE EMPLOYEE drop constraint unEmp ;
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SQL QUERIES
 Not same as the SELECT operation of the relational algebra
 The result can have duplicate tuples
 SQL relation is a multi-set (bag) of tuples; not a set of tuples
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SELECT <attributes>
FROM <one or more relations>
WHERE <conditions>
Basic form:

SIMPLE SQL QUERY
PName Price Category Manufacturer
Gizmo $19.99 Gadgets GizmoWorks
Powergizmo $29.99 Gadgets GizmoWorks
SingleTouch $149.99 Photography Canon
MultiTouch $203.99 Household Hitachi
SELECT *
FROM Product
WHERE category=‘Gadgets’
Product
“selection”

SIMPLE SQL QUERY
SingleTouch $149.99 Photography Canon
MultiTouch $203.99 Household Hitachi
SELECT PName, Price, Manufacturer
FROM Product
WHERE Price > 100
Product
PName Price Manufacturer
SingleTouch $149.99 Canon
MultiTouch $203.99 Hitachi
“selection” and
“projection”

ELIMINATING DUPLICATES
SELECT DISTINCT category
FROM Product
Compare to:
SELECT category
FROM Product
Category
Gadgets
Gadgets
Photography
Household
Category
Gadgets
Photography
Household

RELATIONAL DATABASE SCHEMA
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SIMPLE SQL QUERIES
 Basic SQL queries correspond to using the SELECT, PROJECT, and
JOIN operations of the relational algebra
 Retrieve the birthdate and address of the employee whose name is
'John B. Smith'.
SELECT BDATE, ADDRESS
FROM EMPLOYEE
WHERE FNAME='John' AND MINIT='B’
AND LNAME='Smith’
 Similar to a SELECT-PROJECT pair of relational algebra operations
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JOIN OPERATION
 Retrieve the name and address of all employees who work for the
'Research' department.
SELECT FNAME, LNAME, ADDRESS
FROM EMPLOYEE AS E
JOIN DEPARTMENT AS D ON E.DNO = D.DNUMBER
WHERE D.DNAME = 'Research';
 DNAME='Research’ is a selection condition
 DNUMBER=DNO is a join condition
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JOIN(CONT.)
 For every project located in 'Stafford', list the project number, the controlling
department number, and the department manager's last name, address, and
birthdate.
SELECT P.PNUMBER, P.DNUM, E.LNAME, E.BDATE, E.ADDRESS
FROM PROJECT AS P
JOIN DEPARTMENT AS D ON P.DNUM = D.DNUMBER
JOIN EMPLOYEE AS E ON D.MGRSSN = E.SSN
WHERE P.PLOCATION = 'Stafford';
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UNSPECIFIED WHERE-CLAUSE
 Missing WHERE-clause
 indicates there is no condition and is same as WHERE TRUE
 Retrieve the SSN values for all employees.
SELECT SSN
FROM EMPLOYEE
 If there is no join condition, then we get CARTESIAN PRODUCT
SELECT SSN, DNAME
FROM EMPLOYEE, DEPARTMENT
OR
SELECT E.SSN, D.DNAME
FROM EMPLOYEE AS E
CROSS JOIN DEPARTMENT AS D;
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USE OF *
 To retrieve all the attribute values of the selected tuples, a * is
used, which stands for all the attributes
Examples:
SELECT *
FROM EMPLOYEE
WHERE DNO=5
SELECT *
FROM EMPLOYEE AS E
JOIN DEPARTMENT AS D ON E.DNO = D.DNUMBER
WHERE D.DNAME = 'Research';
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ALIASES
 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 attributes with the same name
must prefix the relation name to the attribute name
Example:
EMPLOYEE.DNO, DEPARTMENT.DNUMBER
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ALIASES
 For each employee, retrieve the employee's name, and the name of his
or her immediate supervisor.
 SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME
FROM EMPLOYEE E S
WHERE E.SUPERSSN=S.SSN
 Can also use the AS keyword to specify aliases
FROM EMPLOYEE AS E
JOIN EMPLOYEE AS S ON E.SUPERSSN = S.SSN;
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ARITHMETIC OPERATIONS
 Arithmetic operators '+', '-'. '*', and '/’) can be applied to
numeric values in an SQL query result
 Give all employees who work on the 'ProductX' project a 10%
raise.
 SELECT E.FNAME, E.LNAME, 1.1 * E.SALARY
 FROM EMPLOYEE AS E
 JOIN WORKS_ON AS W ON E.SSN = W.ESSN
 JOIN PROJECT AS P ON W.PNO = P.PNUMBER
 WHERE P.PNAME = 'ProductX';
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ORDER BY
 The ORDER BY clause sort the tuples in a query result
 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, then first name.
SELECT D.DNAME, E.LNAME, E.FNAME, P.PNAME
FROM DEPARTMENT AS D
JOIN EMPLOYEE AS E ON D.DNUMBER = E.DNO
JOIN WORKS_ON AS W ON E.SSN = W.ESSN
JOIN PROJECT AS P ON W.PNO = P.PNUMBER
ORDER BY D.DNAME, E.LNAME, E.FNAME;
The default order is in ascending order of values
 We can specify the keyword DESC if we want a descending order
 ORDER BY Dname DESC, Lname ASC
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NULLS IN SQL QUERIES
 SQL allows queries that check if a value is NULL
 SQL uses IS or IS NOT to compare NULLs
 Retrieve the names of all employees who do not have
supervisors.
 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
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SUBSTRING COMPARISON
 LIKE 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
 Retrieve all employees whose address is in Houston,
Texas.
FROM EMPLOYEE
WHERE ADDRESS LIKE '%Houston,TX%’
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SUBSTRING COMPARISON (CONT.)
 Retrieve all employees who were born during the 1950s.
 Here, '5' must be the third character of the string , so the
BDATE value is ‘__5_______',.
SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE BDATE LIKE ‘__5_______',.
 LIKE operator allows us to get around the fact that each value
is considered atomic and indivisible;
 hence, in SQL, character string attribute values are not atomic
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JOINED RELATIONS IN SQL
 Allows the user to specify different types of joins (regular "theta" JOIN, NATURAL JOIN,
LEFT OUTER JOIN, RIGHT OUTER JOIN, CROSS JOIN, etc )
 Example:
 SELECTE.FNAME, E.LNAME, S.FNAME, S.LNAME
FROM EMPLOYEE AS E , EMPLOYEE AS S
WHERE E.SUPERSSN=S.SSN
FROM (EMPLOYEE AS E
JOIN EMPLOYEE AS S ON E.SUPERSSN = S.SSN);
FROM EMPLOYEE AS E
LEFT OUTER JOIN EMPLOYEE AS S ON E.SUPERSSN = S.SSN;
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JOINED RELATIONS FEATURE IN SQL
WHERE DNAME='Research' AND DNUMBER=DNO
could be written as:
FROM (EMPLOYEE JOIN DEPARTMENT
ON DNUMBER=DNO)
WHERE DNAME='Research’
or as:
FROM (EMPLOYEE NATURAL JOIN DEPARTMENT
AS DEPT(DNAME, DNO, MSSN, MSDATE)
WHERE DNAME='Research’
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JOINED RELATIONS FEATURE IN SQL
 Example that illustrates multiple joins
SELECT PNUMBER, DNUM, LNAME,
FROM (PROJECT JOIN DEPARTMENT
ON DNUM=DNUMBER)
JOIN EMPLOYEE ON MGRSSN=SSN) )
WHERE PLOCATION='Stafford’
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SET OPERATIONS
 SQL has incorporated some set operations like
 Union operation (UNION),
 Set difference (EXCEPT) and
 Intersection operation (INTERSECT)
 Duplicate tuples are eliminated from the result
 Requires union compatible relations
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SET OPERATIONS (CONT.)
 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.
(SELECT P.PNAME
FROM PROJECT AS P
JOIN DEPARTMENT AS D ON P.DNUM = D.DNUMBER
JOIN EMPLOYEE AS E ON D.MGRSSN = E.SSN
WHERE E.LNAME = 'Smith’)
UNION
(SELECT P.PNAME
FROM PROJECT AS P
JOIN WORKS_ON AS W ON P.PNUMBER = W.PNO
JOIN EMPLOYEE AS E ON W.ESSN = E.SSN
WHERE E.LNAME = 'Smith');
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AGGREGATE FUNCTIONS
 Include COUNT, SUM, MAX, MIN, and AVG
 Find the maximum salary, the minimum salary, and the average
salary among all employees.
SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY)
FROM EMPLOYEE
 Some SQL implementations may not allow more than one function in
the SELECT-clause
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AGGREGATE FUNCTIONS (CONT.)
 Retrieve the number of employees in the 'Research'
department
SELECT COUNT (*)
WHERE DNO=DNUMBER AND
DNAME='Research’
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GROUPING
 GROUP BY-clause specifies the grouping attributes
 For each department, retrieve the department number,
the number of employees in the department, and their
average salary.
 SELECT DNO, COUNT (*), AVG (SALARY)
FROM EMPLOYEE
GROUP BY DNO
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GROUPING (CONT.)
 For each project, retrieve the project number, project
name, and the number of employees who work on that
project.

SELECT PNUMBER, PNAME, COUNT (*)
FROM PROJECT, WORKS_ON
WHERE PNUMBER=PNO
GROUP BY PNUMBER, PNAME
 The grouping and functions are applied after the joining of the two
relations
 Group By clause specifies grouping attributes which should appear
in SELECT clause
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HAVING-CLAUSE
 HAVING-clause specify a selection condition on groups
 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.

SELECT PNUMBER, PNAME, COUNT (*)
FROM PROJECT, WORKS_ON
WHERE PNUMBER=PNO
GROUP BY PNUMBER, PNAME
HAVING COUNT (*) > 2
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GROUP BY AND HAVING
 Count the total number of employees whose salaries
exceed $40,000 in each department, but only for
departments where more than five employees work
 SELECT Dname, COUNT (*)
FROM DEPARTMENT, EMPLOYEE
WHERE Dnumber=Dno AND Salary>40000
GROUP BY Dname
HAVING COUNT (*) > 5;
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GENERAL FORM OF GROUPING AND
AGGREGATION
Evaluation steps:
1. Evaluate FROM-WHERE, apply condition C1
2. Group by the attributes a1,…,ak
3. Apply condition C2 to each group (may have aggregates)
4. Compute aggregates in S and return the result
SELECT S
FROM R1,…,Rn
WHERE C1
GROUP BY a1,…,ak
HAVING C2

TWO EXAMPLES
Store(sid, sname)
Product(pid, pname, price, sid)
Find stores that sell only products with price > 100
same as:
Find stores s.t. all their products have price > 100)

SELECT Store.name
FROM Store, Product
WHERE Store.sid = Product.sid
GROUP BY Store.sid, Store.name
HAVING 100 < min(Product.price)
SELECT Store.name
FROM Store
WHERE Store.sid NOT IN
(SELECT Product.sid
FROM Product
WHERE Product.price <= 100)
SELECT Store.name
FROM Store
WHERE
100 < ALL (SELECT Product.price
FROM product
WHERE Store.sid = Product.sid)
Almost equivalent…
Find stores s.t. all their
products have price > 100

AGGREGATE EXAMPLE
 Example: Count the number of distinct salary values in
the database.
 SELECT COUNT (DISTINCT Salary)
 FROM EMPLOYEE;
 NULL values are discarded when aggregate functions
are applied to a particular attribute.
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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>]
 A query is evaluated by first applying the WHERE-clause,
then GROUP BY and HAVING, and finally the SELECT-clause
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SUMMARY OF SQL QUERIES (CONT.)
 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
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SQL QUERIES
 There are various ways to specify the same query in SQL
 This is to give flexibility to user to specify queries
 For query optimization, it is preferable to write a query
with as little nesting and implied ordering as possible.
 Ideally, DBMS should process the same query in the
same way regardless of how the query is specified.
 But this is quite difficult in practice, (chapter 19,20)
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SPECIFYING UPDATES IN SQL
 There are three SQL commands to modify the database;
 INSERT,
 DELETE, and
 UPDATE
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INSERT
 It is used to add one or more tuples to a relation
 Example:
INSERT INTO EMPLOYEE
VALUES ('Richard','K','Marini', '653298653', '30-DEC-52',
'98 Oak Forest,Katy,TX', 'M', 37000,'987654321', 4 )
 Attribute values should be listed in the same order as
the attributes were specified in the CREATE TABLE
command
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INSERT (CONT.)
 An alternate form of INSERT specifies explicitly the
attribute names that correspond to the values in the
new tuple
 Example: Insert a tuple for a new EMPLOYEE for whom
we only know the FNAME, LNAME, and SSN attributes.
INSERT INTO EMPLOYEE (FNAME, LNAME, SSN)
VALUES ('Richard', 'Marini', '653298653')
 Attributes with NULL values can be left out
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INSERT (CONT.)
 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 Q1, and is loaded with the
information retrieved from the database by the query Q2.
Q1: CREATE TABLE DEPTS_INFO
(DEPT_NAME VARCHAR(10),
NO_OF_EMPS INTEGER,
TOTAL_SAL INTEGER);
Q2: 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 ;
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DELETE
 Removes tuples from a relation
 Tuples are deleted from only one table at a time (unless
CASCADE is specified on a referential integrity constraint)
 Examples:
DELETE FROM EMPLOYEE
WHERE LNAME='Brown’
DELETE FROM EMPLOYEE
WHERE DNO IN (SELECT DNUMBER
FROM DEPARTMENT
WHERE DNAME='Research')
DELETE FROM EMPLOYEE Slide
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UPDATE
 Used to modify attribute values of selected tuples
 Example: Change the location and controlling
department number of project number 10 to 'Bellaire'
and 5, respectively.
UPDATE PROJECT
SET PLOCATION = 'Bellaire', DNUM = 5
WHERE PNUMBER=10
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UPDATE (CONT.)
 Example: Give all employees in the 'Research' department a
10% raise in salary.
UPDATE EMPLOYEE
SET SALARY = SALARY *1.1
WHERE DNO IN (SELECT DNUMBER
FROMDEPARTMENT
WHERE DNAME='Research')
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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
 Retrieve the name and address of all employees who work for the
'Research' department.
SELECT E.FNAME, E.LNAME, E.ADDRESS
FROM EMPLOYEE AS E
WHERE E.DNO IN (SELECT D.DNUMBER
FROM
DEPARTMENT AS D
WHERE
D.DNAME = 'Research'
);
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CORRELATED NESTED QUERIES
 If a condition in the nested query references an attribute of a relation
declared in the outer query , then two queries are said to be correlated
 Retrieve the name of each employee who has a dependent with the same
first name and gender as the employee.
SELECT E.FNAME, E.LNAME
FROM EMPLOYEE AS E
WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT AS D
WHERE E.SEX = D.SEX AND FNAME=DEPENDENT_NAME)
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Nested query is evaluated once for each tuple in outer query

CORRELATED NESTED QUERIES (CONT.)
 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, the query on previous slide can be written as
Retrieve the name of each employee who has a dependent with the
same first name as the employee.
SELECT E.FNAME, E.LNAME
FROM EMPLOYEE E, DEPENDENT D
WHERE E.SSN=D.ESSN AND
E.FNAME=D.DEPENDENT_NAME
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NESTED QUERIES
 Select the Essns of all employees who work the same (project,
hours) combination that ‘John Smith’ (whose Ssn =
‘123456789’) works on.
 SELECT DISTINCT Essn
FROM WORKS_ON
WHERE (Pno, Hours) IN ( SELECT Pno, Hours
FROM WORKS_ON
WHERE Essn=‘123456789’ );
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SQL allows use of tuples of values in comparisons

EXISTS FUNCTION
 EXISTS Function checks whether the result of a nested query is
empty or not
 Retrieve the name of each employee who has a dependent with the
same first name as the employee.
SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE EXISTS (SELECT *
FROM DEPENDENT
WHERE SSN=ESSN AND
FNAME=DEPENDENT_NAME)
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EXISTS FUNCTION (CONT.)
 Retrieve the names of employees who have no dependents.
SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE NOT EXISTS (SELECT *
FROM DEPENDENT
WHERE SSN=ESSN)
 The above correlated nested query retrieves all DEPENDENT tuples
related to an EMPLOYEE tuple.
 If none exist , the EMPLOYEE tuple is selected
 EXISTS is necessary for the expressive power of SQL
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 Find the names of managers who have at least one
dependents.
SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE EXISTS (SELECT *
FROM DEPENDENT
WHERE SSN=ESSN)
AND
EXISTS (SELECT *
FROM DEPARTMENT
WHERE SSN=Mgr_SSN)
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 Retrieve the name of each employee who works on all
the projects controlled by department number 5.
 Set theory: S1 contains S2 if (S2 – S1 = 0)
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SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE NOT EXISTS (
(SELECT PNUMBER
FROM PROJECT
WHERE DNUM=5)
EXCEPT
(SELECT PNO
FROM WORKS_ON
WHERE SSN=ESSN)
)
S1 = set of projects
of each employee
S2= set of DN0=5
projects

NESTED QUERIES
 Retrieve the name of each employee who works on all the
projects controlled by department number 5.
FROM EMPLOYEE
WHERE ( (SELECT PNO
FROM WORKS_ON
WHERE SSN=ESSN)
CONTAINS
(SELECT PNUMBER
FROM PROJECT
WHERE DNUM=5) ) Slide
8-70
CONTAINS compares two sets , and returns TRUE if one set contains all values in
the other set.
Same as division operation of relational algebra
Most implementations of SQL do not have this operator

NESTED CORRELATED QUERIES (CONTD)
SELECT name
FROM Product
WHERE price > ALL (SELECT price
FROM Purchase
WHERE maker=‘IBM’)
Product ( pname, price, category, maker)
Find products that are more expensive than all those produced
By “IBM”
You can also use: s > ALL R
s > ANY R
EXISTS R

NESTED CORRELATED QUERIES (CONTD)
SELECT Fname
FROM Employee
WHERE Salary > ALL (SELECT Salary
FROM Employee
where Dno=5)
Find Employee whose salary is greater than the salary
of all employee in department 5
You can also use: s > ALL R
s > ANY R
EXISTS R

COMPLEX CORRELATED QUERY
Product ( pname, price, category, maker, year)
 Find products (and their manufacturers) that are more expensive
than all products made by the same manufacturer before 1972
Very powerful ! Also much harder to optimize.
SELECT DISTINCT pname, maker
FROM Product AS x
WHERE price > ALL (SELECT price
FROM Product AS y
WHERE x.maker = y.maker AND x.pname!=y.pname
and
y.year < 1972)

COMPLEX CORRELATED QUERY
 Find Employee (his dno and salary) whose salary is greater than all
employees in the same department
SELECT Fname, Salary, Dno
FROM Employee as E
WHERE Salary > ALL (SELECT Salary
FROM Employee as S
WHERE E.dno=S.dno and E.ssn !=S.ssn )

EXPLICIT SETS
 It is also possible to use an explicit (enumerated) set of
values in the WHERE-clause rather than a nested query
 Retrieve the social security numbers of all employees
who work on project number 1, 2, or 3.
SELECT DISTINCT ESSN
FROM WORKS_ON
WHERE PNO IN (1, 2, 3)
Slide
8-75

Chapter 9-76
VIEWS IN SQL
 A view is a “virtual” table that is derived from other
tables
 Allows for limited update operations (since the table
may not physically be stored)
 Allows full query operations
 A convenience for expressing certain operations
 They are used to:
• simplify complex queries, and
• define distinct conceptual interfaces for different users.

Chapter 9-77
SQL VIEWS: AN EXAMPLE
 Specify a different WORKS_ON table
CREATE VIEW WORKS_ON1 AS
SELECT FNAME, LNAME, PNAME, HOURS
FROM EMPLOYEE, PROJECT, WORKS_ON
WHERE SSN=ESSN AND PNO=PNUMBER

SQL VIEWS: AN EXAMPLE2
Slide
8-78

Chapter 9-79
USING A VIRTUAL TABLE
 We can specify SQL queries on a newly created view:
SELECT FNAME, LNAME
FROM WORKS_ON1
WHERE PNAME=‘ProductX’;
 DBMS is responsible to keep view always up-to-date
 When no longer needed, a view can be dropped:
DROP WORKS_ON1;

Chapter 9-80
EFFICIENT VIEW IMPLEMENTATION
 Query modification: present the view query in terms of a query
on the underlying base tables
SELECT FNAME, LNAME, PNAME, HOURS
FROM EMPLOYEE, PROJECT, WORKS_ON
WHERE SSN=ESSN AND PNO=PNUMBER AND PNAME=‘PRODUCTX’
Disadvantage:
 Inefficient for views defined via complex queries (esp if additional
queries are to be applied within a short time period )

Chapter 9-81
EFFICIENT VIEW IMPLEMENTATION
 View materialization: involves physically creating and
keeping a temporary table
 assumption: other queries on the view will follow
 concerns: maintaining correspondence between the base
table and the view when the base table is updated
 strategy: incremental update

Chapter 9-82
VIEW UPDATE
 Single view without aggregate operations:
 update may map to an update on the underlying
base table
 Views involving joins:
 an update may map to an update on the underlying
base relations
 not always possible
 Example:
UPDATE WORKS_ON1
SET PNAME=XYZ'
WHERE FNAME=‘JOHN AND
LNAME='SMITH' AND PNAME=‘PRODUCTX’

A) UPDATE WORKS_ON1
SET PNO = (SELECT PNUMBER FROM PROJECT
WHERE PNAME=‘XYZ’)
WHERE ESSN IN (SELECT SSN FROM EMPLOYEE
WHERE LNAME=‘SMITH’ AND FNAME=‘JOHN’)
AND
PNO = (SELECT PNUMBER FROM PROJECT
WHERE PNAME=‘PRODUCTX’)
B)UPDATE PROJECT SET PNAME=‘XYZ’
WHERE PNAME=‘PRODUCTX’
Slide
8-83

Chapter 9-84
UN-UPDATABLE VIEWS
 Views defined using groups and aggregate functions are
not updateable
 Views defined on multiple tables using joins are generally not
updateable
 WITH CHECK OPTION: must be added to the definition of a
view if the view is to be updated
 to allow check for updatability and to plan for an execution
strategy

ASSERTION
 In SQL, table constraints are associated with a single table.
 CHECK(DNUMBER >0 AND DNUMBER <25)
 CHECK (Dept_create_date <= Mgr_start_date)
 Expression in the CHECK clause can refer to other tables but when a
constraint involves many tables, it becomes cumbersome
 Assertions are constraints that are not associated with any one table.
 SQL-server do not support assertion, however it do support
trigger and check constraint
Slide
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Chapter 9-86
ASSERTIONS: AN EXAMPLE
 “The salary of an employee must not be greater than the
salary of the manager of the department that the employee
works for’’
CREATE ASSERTION SALARY_CONSTRAINT
CHECK (NOT EXISTS (SELECT *
FROM EMPLOYEE E, EMPLOYEE M, DEPARTMENT
D
WHERE E.SALARY > M.SALARY AND
E.DNO=D.NUMBER AND D.MGRSSN=M.SSN))

Chapter 9-87
USING GENERAL ASSERTIONS
 Specify a query that violates the condition; include
inside a NOT EXISTS clause
 Query result must be empty
 if the query result is not empty, the assertion has been
violated
 Create Assertion to enforce the constraint that
 The number of Employees in each department should be less than 20.
 An employee should be assign a project of hisher department only.
 The total working hours of all the employees in a department should be
less than 180.

 CHECK clause can also be used to specify constraints on
individual attributes, domains and individual tuples
CONSTRAINT deptPK PRIMARY KEY(Dnumber)
CONSTRAINT deptSK UNIQUE(Dname)
CHECK (Dept_create_date <= Mgr_start_date)
 Difference between CREATE ASSERTION and the
individual domain and tuple constraints is
 CHECK on individual attributes, domains, and tuples are
checked only when tuples are inserted or updated.
Slide
8-88

Chapter 9-89
SQL TRIGGERS
 Objective: to monitor a database and take action when a
condition occurs
 Triggers are expressed in a syntax similar to assertions
and include the following:
 event (e.g., an update operation)
 condition
 action (to be taken when the condition is satisfied)

Chapter 9-90
SQL TRIGGERS: AN EXAMPLE
 A trigger to compare an employee’s salary to his/her
supervisor during insert or update operations:
CREATE TRIGGER INFORM_SUPERVISOR
BEFORE INSERT OR UPDATE OF
SALARY, SUPERVISOR_SSN ON EMPLOYEE
FOR EACH ROW
WHEN
(NEW.SALARY> (SELECT SALARY FROM EMPLOYEE
WHERE SSN=NEW.SUPERVISOR_SSN))
INFORM_SUPERVISOR
(NEW.SUPERVISOR_SSN,NEW.SSN;

EXAMPLE: BOAT RENTAL
DATABASE
 Consider the following Boat Rental database schema
 SAILOR (SID, SName, Phone, City)
 BOAT (BName, BType, Price, OID)
 RESERVATION (SID, BName, Date, Duration)
 OWNER (OID, OName, Phone, Street, City, Country)
 Find the name and city of sailors who reserve every
‘Fishing’ boat.
 Find the names of boats that are reserved by at least ten
different sailors.
 List names, owner names, and prices of the boats which
were reserved in 2007 but not in 2008.
Slide
8-91

DATABASE
 Consider the following schema
 Find the name and city of sailors who reserve every ‘Fishing’ boat.
 SELECT SNAME, CITY
FROM SAILOR s
WHERE NOT EXISTS ( (SELECT BNAME
FROM BOAT
WHERE BTYPE=’Fishing’)
EXCEPT
(SELECT BNAME
FROM reservation r
WHERE r.SID=s.SID) )
Slide
8-92

DATABASE
 Find the names of boats that are reserved by at least ten
different sailors.
 Select bname
From reservation r
Group by bname
Having count(DISTINCT SID) >9
Slide
8-93

DATABASE
 List names, owner names, and prices of the boats which were reserved in
2007 but not in 2008.
 Select distinct b.bname, b.price, o.oname
From reservation r, boat b, owner o
Where r.bname = b.bname and b.oid=o.oid and r.date LIKE '%2007%'
and r.sid not in
(Select ic.sid
From reservation ic, boat ip
Where ic.bname = ip.bname and ip.date LIKE '%2008%' )
Slide
8-94

EXAMPLE: BOAT RENTAL DATABASE
 Consider the following Boat Rental database schema:
 SELECT DISTINCT Bname
 FROM BOAT
 WHERE Price > ALL (SELECT price
FROM BOAT b , OWNER o
WHERE b.oid=o.oid and Country=‘Pakistan’ )
What does the query do?

EXAMPLE: BOAT RENTAL DATABASE
 Consider the following Boat Rental database schema:
 Select bname,count(*)
 From reservation r ,boat b,owner o
 Where b.bname=r.bname and b. oid=o.oid and country=‘Pakistan’
 Group by bname
 Having count(*) > 5
What does the query do?

SQL database management System a brief intro

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