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Fundamentals of
Database Systems
Chapter 8
SQL99 - The Relational Database
Standard
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Data Definition in SQL
 Schema and Catalog Concepts in SQL2
 An SQL schema is identified by
» A schema name
» An authorization identifier to indicate the user or
account who owns the schema
» Descriptors for each element in the schema
 Schema elements include the tables,
constraints, views, domains, and other
constructs (such as authorization grants) that
describe the schema
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Data Definition in SQL (cont.)
 A schema is created via the CREATE
SCHEMA statement
» Example
Create a schema called COMPANY, owned by the
user with authorization identifier JSMITH:
CREATE SCHEMA COMPANY
AUTHORIZATION JSMITH;
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Data Definition in SQL (cont.)
 Catalog
» A named collection of schemas in an SQL
environment
» A catalog always contains a special schema
called INFORMATION_SCHEMA, which
provides information on all the element
descriptors of all the schemas in the catalog to
authorized users
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Data Definition in SQL (cont.)
 Purpose
 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
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Data Definition in SQL (cont.)
 Data types in SQL2
» Numeric
– INT or INTEGER
– SMALLINT
– FLOAT
– REAL
– DOUBLE PRECISION
– DEC(i, j) or DECIMAL(i,j) or NUMERIC(i,j)
» Character-string
– CHAR(n) or CHARACTER(n)
– VARCHAR(n) or CHAR VARYING(n) or
CHARACTER VARYING(n)
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Data Definition in SQL (cont.)
» Bit-string
– BIT(n)
– BIT VARYING(n)
» Date
– In the form YYYY-MM-DD
» Time
– In the form 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
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Data Definition in SQL (cont.)
» TIMESTAMP
– Has both DATE and TIME components
» INTERVAL
– Specifies a relative value rather than an absolute value
– Can be DAY/TIME or YEAR/MONTH intervals
– Can be positive or negative
– when added to or subtracted from an absolute value,
the result is an absolute value
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Data Definition in SQL (cont.)
 In SQL2, it is possible to specify the domain
of each attribute and declare the domain
name, e.g.,
CREATE DOMAIN SSN_TYPE AS CHAR(9);
» We can use SSN_TYPE in place of CHAR(9)
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Data Definition in SQL (cont.)
 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) );
 In SQL2, the CREATE TABLE can be used
to specify the primary key, secondary keys,
and referential integrity constraints (foreign
keys)
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Data Definition in SQL (cont.)
» Primary key attributes can be specified via the
PRIMARY KEY
» Key attributes can be specified via UNIQUE
» Referential integrity is specified via the
FOREIGN KEY clause
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);
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Data Definition in SQL (cont.)
 It is also possible to define a default value for
an attribute by appending the clause
DEFAULT <value> to an attribute definition
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Data Definition in SQL (cont.)
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Data Definition in SQL (cont.)
 In SQL2, action can be specified if a
referential integrity constraint is violated
upon deletion of a referenced tuple or upon
modification of a referenced primary key
» Attaching a referential triggered action clause to
any foreign key constraint
» The options include SET NULL, CASCADE,
and SET DEFAULT
» An option must be qualified with either ON
DELETE or ON UPDATE
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Data Definition in SQL (cont.)
» SET NULL (DEFAULT) ON DELETE: the value
of the deleted referencing attributes is changed to
NULL or the specified default value
» SET NULL (DEFAULT) ON UPDATE: the value
of the updated referencing attributes is changed
to NULL or the specified default value
» CASCADE ON DELETE: delete all the
referencing tuples
» CASCADE ON UPDATE: change the value of
the foreign key to the updated (new) primary key
value for all referencing tuples
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Data Definition in SQL (cont.)
» A constraint may be given a name, following the
keyword CONSTRAINT
» The names of all constraints within a particular
schema must be unique
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Data Definition in SQL (cont.)
 Base tables (or base relations)
» The relations declared through CREATE TABLE
statements are called
» The relation and its tuples are actually created
and stored as a file by the DBMS
 Virtual relations
» The relations created through the CREATE
VIEW statement
» The relations may or may not correspond to an
actual physical file
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Data Definition in SQL (cont.)
 DROP SCHEMA
 Used to a whole schema
 There are two drop behavior options
» CASCADE: remove the database schema and all
its tables, domains, and other elements, e.g.,
DROP SCHEMA COMPANY CASCADE;
» RESTRICT: the schema is dropped only if it has
no elements
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Data Definition in SQL (cont.)
 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
DROP TABLE DEPENDENT;
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Data Definition in SQL (cont.)
 Two options
» CASCADE: all constraints and views that
reference the table are dropped automatically
along with the table itself
DROP TABLE DEPENDENT CASCADE;
» RESTRICT: a table is dropped only if it is not
referenced in any constraints or views
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Data Definition in SQL (cont.)
 ALTER TABLE
 Used to change the base relation definition
 Possible actions include
» adding or dropping a column (attribute)
» changing a column definition
» adding or dropping table constraints
 Example
ALTER TABLE COMPANY.EMPLOYEE
ADD JOB VARCHAR(12);
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Characteristics of Relations
» The new attribute will have NULLs in all the
tuples of the relation
» Hence, the NOT NULL constraint is not
allowed for such an attribute
» 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
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Characteristics of Relations
 To drop a column, we must choose either
CASCADE or RESTRICT for drop behavior
» CASCADE: all constraints and views that
reference the column are dropped automatically
along with the column
» RESTRICT: the column is drop only if no views
or constraints reference the column
» Example
ALTER TABLE COMPANY.EMPLOYEE DROP
ADDRESS CASCADE;
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Characteristics of Relations
 It is also possible to alter a column definition
by dropping an existing default clause or by
defining a new default clause
» Example 1
ALTER TABLE COMPANY.DEPARTMENT
ALTER MGRSSN DROP DEFAULT;
» Example 2
ALTER TABLE COMPANY.DEPARTMENT
ALTER MGRSSN SET DEFAULT "333445555";
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Retrieval Queries in SQL
 Preliminary
 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
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Retrieval Queries in SQL (cont.)
 Important distinction between SQL and the
formal relational model
» SQL allows a table (relation) to have two or more
identical tuples
» 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
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Retrieval Queries in SQL (cont.)
 Basic form of the SQL SELECT statement
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
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Retrieval Queries in SQL (cont.)
 Simple SQL Queries
 Basic SQL queries correspond to using the
SELECT, PROJECT, and JOIN operations of
the relational algebra
 All subsequent examples use the COMPANY
database
 Example of a simple query on one relation
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Retrieval Queries in SQL (cont.)
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Retrieval Queries in SQL (cont.)
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Retrieval Queries in SQL (cont.)
 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
– the WHERE-clause specifies the selection condition
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Retrieval Queries in SQL (cont.)
 BDATE,ADDRESS (FNAME=‘John’ AND MINIT=‘B’ AND
LNAME=‘Smith’ (EMPLOYEE))
» However, the result of the query may contain
duplicate tuples
» Result of Q0
BDATE
ADDRESS
1965-01-09
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731 Fondren, Houston, TX
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Retrieval Queries in SQL (cont.)
 Query 1: Retrieve the name and address of all
employees who work for the 'Research'
department
Q1:
SELECT
FROM
WHERE
FNAME, LNAME, ADDRESS
EMPLOYEE, DEPARTMENT
DNAME='Research' AND
DNUMBER=DNO;
» Similar to a SELECT-PROJECT-JOIN sequence of
relational algebra operations
– (DNAME='Research') is a selection condition
– (DNUMBER=DNO) is a join condition
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Retrieval Queries in SQL (cont.)
 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
FROM
WHERE
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PNUMBER, DNUM, LNAME,
BDATE, ADDRESS
PROJECT, DEPARTMENT,
EMPLOYEE
DNUM=DNUMBER AND
MGRSSN=SSN AND
PLOCATION='Stafford’;
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Retrieval Queries in SQL (cont.)
» 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
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Retrieval Queries in SQL (cont.)
 Aliases (Renaming) with SQL
 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
» e.g., EMPLOYEE.LNAME,
DEPARTMENT.DNAME
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Retrieval Queries in SQL (cont.)
 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
FROM
WHERE
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E.FNAME, E.LNAME,
S.FNAME, S.LNAME
EMPLOYEE E S
E.SUPERSSN=S.SSN;
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Retrieval Queries in SQL (cont.)
» In Q8, the alternate relation names E and S are
called aliases for the EMPLOYEE relation
» We can think of E and S as two different copies of
the EMPLOYEE relation
– E represents employees in the role of supervisees
– S represents employees in the role of supervisors
» We an also use the AS keyword to specify aliases,
e.g., for Q8
FROM EMPLOYEE E S
FROM
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EMPLOYEE AS E, EMPLOYEE AS S
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Retrieval Queries in SQL (cont.)
» Aliasing can also be used in any SQL query for
convenience
Q1B: SELECT E.FNAME, E.NAME,
E.ADDRESS
FROM
EMPLOYEE E,
DEPARTMENT D
WHERE D.NAME=‘Research’ AND
D.DNUMBER=E.DNUMBER;
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Retrieval Queries in SQL (cont.)
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 WHERE TRUE
 Query 9: Retrieve the SSN values for all
employees
Q9:
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SELECT
FROM
SSN
EMPLOYEE;
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Retrieval Queries in SQL (cont.)
 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
» Query 10: Retrieve all combinations of
EMPLOYEE SSN and DEPARTMENT DNAME
Q10: SELECT SSN, DNAME
FROM
EMPLOYEE, DEPARTMENT;
» It is extremely important not to overlook
specifying any selection and join conditions in the
WHERE-clause
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Retrieval Queries in SQL (cont.)
Use of *
 To retrieve all the attribute values of the
selected tuples, a * is used, which stands for
all the attributes
 Query 1C: Retrieves all the attribute values of
EMPLOYEE at DEPARTMENT number 5
Q1C: SELECT
FROM
WHERE
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*
EMPLOYEE
DNO=5;
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Retrieval Queries in SQL (cont.)
 Query Q1D: Retrieves all the attributes of an
EMPLOYEE and the attributes of the
DEPARTMENT he or she works in, for every
employee of the ‘Research’ department
Q1D: SELECT
FROM
WHERE
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*
EMPLOYEE, DEPARTMENT
DNAME='Research' AND
DNO=DNUMBER;
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Retrieval Queries in SQL (cont.)
 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
Q11: SELECT
FROM
Q11A: SELECT
FROM
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SALARY
EMPLOYEE;
DISTINCT SALARY
EMPLOYEE;
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Retrieval Queries in SQL (cont.)
» The result of Q11 may have duplicate SALARY
values whereas Q11A does not have any
duplicate values
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Retrieval Queries in SQL (cont.)
 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; duplicates are eliminated
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Retrieval Queries in SQL (cont.)
 The set operations apply only to union
compatible relations
 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
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Retrieval Queries in SQL (cont.)
Q4:
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(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 LNAME='Smith');
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Retrieval Queries in SQL (cont.)
 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
» '_' replaces a single arbitrary character
 Query 12: Retrieve all employees whose
address is in Houston, Texas
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Retrieval Queries in SQL (cont.)
Q12: SELECT FNAME, LNAME
FROM
EMPLOYEE
WHERE ADDRESS LIKE '%Houston,TX%’;
 Query 12A: Retrieve all employees who were
born during the 1950s
Q12A: SELECT FNAME, LNAME
FROM
EMPLOYEE
WHERE BDATE LIKE '__5_______’;
» Here, '5' must be the 3th character of the string
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Retrieval Queries in SQL (cont.)
 Arithmetic Operations
 The standard arithmetic operators '+', '-'. '*',
and '/' can be applied to numeric values in
an SQL query result
 Query 13: Show the effect of giving all
employees who work on the 'ProductX'
project a 10% raise
Q13: SELECT FNAME, LNAME, 1.1*SALARY
FROM
EMPLOYEE, WORKS_ON, PROJECT
WHERE SSN=ESSN AND PNO=PNUMBER
AND PNAME='ProductX’;
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Retrieval Queries in SQL (cont.)
 ORDER BY
 The ORDER BY clause is used to sort the
tuples in a query result based on the values of
some attribute(s)
 The default order is in ascending order
 Query 15: 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
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Retrieval Queries in SQL (cont.)
Q15:SELECT
DNAME, LNAME, FNAME,
PNAME
FROM
DEPARTMENT, EMPLOYEE,
WORKS_ON, PROJECT
WHERE
DNUMBER=DNO AND SSN=ESSN
AND PNO=PNUMBER
ORDER BY DNAME, LNAME, FNAME;
 We can specify the keyword DESC if we
want a descending order
ORDER BY DNAME DESC, LNAME ASC,
FNAME ASC
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More Complex Queries
 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
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More Complex Queries (cont.)
Q1: SELECT
FROM
WHERE
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FNAME, LNAME, ADDRESS
EMPLOYEE
DNO IN
(SELECT DNUMBER
FROM
DEPARTMENT
WHERE DNAME='Research');
» 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
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More Complex Queries (cont.)
» 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
» In this example, the nested query is not
correlated with the outer query
 A reference to an unqualified attribute refers
to the relation declared in the innermost
nested query
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More Complex Queries (cont.)
 Query 16: Retrieve the name of each
employee who has a dependent with the same
first name and same sex as the employee
Q16: SELECT E.FNAME, E.LNAME
FROM
EMPLOYEE AS E
WHERE E.SSN IN
(SELECT ESSN
FROM
DEPENDENT
WHERE E.FNAME=DEPENDENT_NAME
AND E.SEX=SEX);
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More Complex Queries (cont.)
 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
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More Complex 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, Q16 may be written as in Q16A
Q16A: SELECT E.FNAME, E.LNAME
FROM EMPLOYEE AS E, DEPENDENT AS D
WHERE E.SSN = D.ESSN AND E.SEX = D.SEX
AND E.FNAME=D.DEPENDENT_NAME;
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More Complex Queries (cont.)
 The EXISTS Function
 EXISTS is used to check whether the result
of a correlated nested query is empty or not
 We can formulate Query 16 in an alternative
form that uses EXISTS as Q16B below
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More Complex Queries (cont.)
Q16: SELECT E.FNAME, E.LNAME
FROM
EMPLOYEE AS E
WHERE EXISTS
(SELECT *
FROM
DEPENDENT
WHERE E.SSN = ESSN AND
E.SEX = SEX AND
E.FNAME = DEPENDENT_NAME);
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More Complex Queries (cont.)
 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
» If none exist, EMPLOYEE tuple is selected
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More Complex Queries (cont.)
 Explicit Sets
 It is also possible to use an explicit
(enumerated) set of values in the WHEREclause rather than a nested query
 Query 17: Retrieve the social security
numbers of all employees who work on
project number 1, 2, or 3
Q13: SELECT
FROM
WHERE
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DISTINCT ESSN
WORKS_ON
PNO IN (1, 2, 3);
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More Complex Queries (cont.)
 NULLS in SQL Queries
 SQL allows queries that check if a value is
NULL (missing or undefined or not
applicable)
 Any comparison with null returns unknown
» E.g.
5 < null or null <> null or
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null = null
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More Complex Queries (cont.)
 Three-valued logic using the truth value unknown:
 OR: (unknown or true) = true, (unknown or false) =
unknown, (unknown or unknown) = unknown
 AND: (true and unknown) = unknown, (false and
unknown) = false, (unknown and unknown) =
unknown
 NOT: (not unknown) = unknown
 “P is unknown” evaluates to true if predicate P
evaluates to unknown
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More Complex Queries (cont.)
 SQL uses IS or IS NOT to compare NULLs
because it considers each NULL value
distinct from other NULL values
 Query 18: Retrieve the names of all
employees who do not have supervisors
Q18: SELECT
FROM
WHERE
» Result of Q18
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FNAME, LNAME
EMPLOYEE
SUPERSSN IS NULL;
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More Complex Queries (cont.)
 Aggregate Functions
 Include COUNT, SUM, MAX, MIN, and
AVG
 Query 19: Find the sum of salaries, the
maximum salary, the minimum salary, and
the average salary among all employees
Q19: SELECT SUM(SALARY), MAX(SALARY),
MIN(SALARY), AVG(SALARY)
FROM EMPLOYEE;
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More Complex Queries (cont.)
 Queries 21 and 22: Retrieve the total number
of employees in the company (Q21), and the
number of employees in the 'Research'
department (Q22)
Q21: SELECT
FROM
COUNT (*)
EMPLOYEE;
Q22: SELECT
COUNT (*)
FROM
WHERE
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EMPLOYEE, DEPARTMENT
DNO=DNUMBER AND
DNAME='Research’;
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More Complex Queries (cont.)
 Grouping
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 In many cases, we want to apply the
aggregate functions to subgroups of tuples in
a relation
 Each subgroup consists of the tuples having
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
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More Complex Queries (cont.)
 Query 24: For each department, retrieve the
department number, the number of
employees in the department, and their
average salary
Q24: SELECT
DNO, COUNT (*),
AVG (SALARY)
FROM
EMPLOYEE
GROUP BY
DNO;
» In Q24, the EMPLOYEE are divided into groups
having the same value for attribute DNO
» The COUNT and AVG functions are applied to
each subgroup of tuples separately
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More Complex Queries (cont.)
 Illustration of Q24
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More Complex Queries (cont.)
 Query 25: For each project, retrieve the
project number, project name, and the number
of employees working on that project
Q25: 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
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More Complex Queries (cont.)
 The HAVING-CLAUSE
 Sometimes we want to retrieve the values of
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)
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More Complex Queries (cont.)
 Query 26: For each project on which more
than two employees work, retrieve the project
number, project name, and the number of
employees
Q26: SELECT
PNUMBER, PNAME,
COUNT (*)
FROM
PROJECT, WORKS_ON
WHERE
PNUMBER=PNO
GROUP BY PNUMBER, PNAME
HAVING
COUNT (*) > 2;
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More Complex Queries (cont.)
 Illustration of Query 26
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More Complex Queries (cont.)
 Illustration of Query 26
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More Complex Queries (cont.)
 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, ordered as follows
SELECT <attribute list>
FROM
<table list>
[WHERE <condition>]
[GROUP BY <grouping attribute(s)>]
[HAVING <group condition>]
[ORDER BY <attribute list>]
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More Complex Queries (cont.)
 A query is evaluated by
» first applying the FROM-clause,
» followed by the WHERE-clause,
» then GROUP BY and HAVING, and
» finally the SELECT-clause
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Specifying Updates in SQL
 SQL commands to modify the database
 INSERT, DELETE, and UPDATE
 INSERT Command
 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
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Specifying Updates in SQL (cont.)
 Example: Add a new tuple to the EMPLOYEE
relation
U1:
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INSERT INTO EMPLOYEE
VALUES
('Richard','K','Marini',
'653298653', '30-DEC-52',
'98 Oak Forest,Katy,TX',
'M', 37000,'987654321', 4);
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Specifying Updates in SQL (cont.)
 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')
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Specifying Updates in SQL (cont.)
 Note: Only the constraints specified in the DDL
commands are automatically enforced by the
DBMS when updates are applied to the database
 If a system does not support some constraint,
the users must enforce the constraint
U2: INSERT INTO EMPLOYEE (FNAME,
LNAME, SSN, DNO)
VALUES (‘Robert’, ‘Hatcher’, ‘980760540’, 2);
(* U2 is rejected if referential integrity checking is
provided by DBMS *)
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Specifying Updates in SQL (cont.)
 A DBMS enforcing NOT NULL will reject an
INSERT command in which an attribute
declared to be NOT NULL does not have a
value
U2A: INSERT INTO EMPLOYEE (FNAME,
LNAME, DNO)
VALUES (‘Robert’, ‘Hatcher’, 5);
(* U2A is rejected if NOT NULL checking is
provided by DBMS *)
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Specifying Updates in SQL (cont.)
 Another variation of INSERT allows insertion
of multiple tuples resulting from a query into a
relation
 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
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Specifying Updates in SQL (cont.)
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 ;
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Specifying Updates in SQL (cont.)
 DELETE Command
 Removes tuples from a relation
 Includes a WHERE-clause to select the tuples to
be deleted
 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
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Specifying Updates in SQL (cont.)
 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
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Specifying Updates in SQL (cont.)
 UPDATE Command
 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
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Specifying Updates in SQL (cont.)
 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
 Example: Give all employees in the 'Research'
department a 10% raise in salary.
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Specifying Updates in SQL (cont.)
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
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Relational Views in SQL
 Basic concepts
 A view is a single virtual table that is derived
from other tables, which could be base tables or
previously defined views
 A view does not necessarily exist in physical
form; that means the view does not store any
tuples
 There are limitations on possible update
operations that can be applied to views
 There is no limitation on querying a view
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Relational Views in SQL (cont.)
 Specify Views
 Using CREATE VIEW command
 The view attribute names can be inherited from
the attribute names of the tables in the defining
query
 Examples:
V1:CREATE
SELECT
FROM
WHERE
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VIEW
WORKS_ON1 AS
FNAME, LNAME, PNAME, HOURS
EMPLOYEE, PROJECT, WORKS_ON
SSN=ESSN AND PNO=PNUMBER;
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Relational Views in SQL (cont.)
V2:CREATE VIEW DEPT_INFO
(DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) AS
SELECT
DNAME, COUNT (*), SUM (SALARY)
FROM
DEPARTMENT, EMPLOYEE
WHERE
DNUMBER=DNO
GROUP BY DNAME;
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Relational Views in SQL (cont.)
 Queries on Views
 A view can be defined to simplify frequently
occurring queries
 Example: Retrieve the last name and first name
of all employees who work on 'ProjectX'.
QV1: SELECT PNAME, FNAME, LNAME
FROM
WORKS_ON1
WHERE PNAME='ProjectX';
» Without the view WORKS_ON1, this query
specification would require two join conditions
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Relational Views in SQL (cont.)
 A view is always up-to-date
» If the base tables on which the view is defined are
modified, the DBMS is responsible for keeping it up
to date
» Hence, the view is not realized at the time of view
definition, but rather at the time we specify a query on
the view
 A view is removed using the DROP VIEW
command
» Example:
V1A: DROP VIEW WORKS_ON1;
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Relational Views in SQL (cont.)
 Updating of Views
 A view update operation may be mapped in
multiple ways to update operations on the
defining base relations
 The topic of updating views is still an active
research area
 Example: Suppose we issue the command in
UV1 to update the WORKS_ON1 view by
modifying the PNAME attribute of 'John
Smith' from 'ProductX' to 'ProductY'.
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Relational Views in SQL (cont.)
UV1: UPDATE
SET
WHERE
WORKS_ON1
PNAME = 'ProductY'
LNAME='Smith' AND FNAME='John'
AND PNAME='ProductX’
» This can be mapped into several updates on the base
relations to give the desired update on the view
» Two possibilities
1. Change the name of the 'ProductX' tuple in the
PROJECT relation to 'ProductY’
2. Relate 'John Smith' to the 'ProductY' PROJECT
tuple in place of the 'ProductX' PROJECT tuple
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Relational Views in SQL (cont.)
» Method 1
UPDATE PROJECT
SET
PNAME = 'ProductY'
WHERE PNAME = 'ProductX’
– This has the effect of changing all the view tuples
with PNAME = ‘ProductX’
– It is quite unlikely that the user who specified
view update UV1 wants the update to be
interpreted this way
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Relational Views in SQL (cont.)
» Method 2
UPDATE WORKS_ON
SET
PNO = (SELECT PNUMBER
FROM
PROJECT
WHERE PNAME='ProductY')
WHERE ESSN IN (SELECT SSN
FROM
EMPLOYEE
WHERE LNAME='Smith'
AND
FNAME='John')
AND
PNO IN (SELECT PNUMBER
FROM
PROJECT
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Relational Views in SQL (cont.)
 Some view updates may not make much sense
» Example: Modify the TOTAL_SAL attribute of
DEPT_INFO as in UV2
UV2:
UPDATE DEPT_INFO
SET
TOTAL_SAL=100000
WHERE DNAME='Research';
 In general, we cannot guarantee that any view
can be updated
 A view update is feasible only if one update on
the base relations can accomplish the desired
update effect on the view
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Relational Views in SQL (cont.)
 If a view update can be mapped to more than
one update on the underlying base relations, we
have to choose the desired update
 General guideline
» A view with a single defining table is updatable if
the view attributes contain the primary key
» Views defined on multiple tables using joins are
generally not updatable
» Views defined aggregate functions are not updatable
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