Cơ sở dữ liệu - Chapter 4: Advanced SQL
The SQL standard defines embeddings of SQL in a variety of
programming languages such as C, Java, and Cobol.
■ A language to which SQL queries are embedded is referred to as a host
language, and the SQL structures permitted in the host language
comprise embedded SQL.
■ The basic form of these languages follows that of the System R
embedding of SQL into PL/I.
■ EXEC SQL statement is used to identify embedded SQL request to the
preprocessor
EXEC SQL
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Database System Concepts, 5th Ed.
©Silberschatz, Korth and Sudarshan
See www.dbbook.com for conditions on reuse
Chapter 4: Advanced SQL
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Chapter 4: Advanced SQL
n SQL Data Types and Schemas
n Integrity Constraints
n Authorization
n Embedded SQL
n Dynamic SQL
n Functions and Procedural Constructs**
n Recursive Queries**
n Advanced SQL Features**
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Builtin Data Types in SQL
n date: Dates, containing a (4 digit) year, month and date
l Example: date ‘2005727’
n time: Time of day, in hours, minutes and seconds.
l Example: time ‘09:00:30’ time ‘09:00:30.75’
n timestamp: date plus time of day
l Example: timestamp ‘2005727 09:00:30.75’
n interval: period of time
l Example: interval ‘1’ day
l Subtracting a date/time/timestamp value from another gives an
interval value
l Interval values can be added to date/time/timestamp values
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Buildin Data Types in SQL (Cont.)
n Can extract values of individual fields from date/time/timestamp
l Example: extract (year from r.starttime)
n Can cast string types to date/time/timestamp
l Example: cast as date
l Example: cast as time
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
UserDefined Types
n create type construct in SQL creates userdefined type
create type Dollars as numeric (12,2) final
n create domain construct in SQL92 creates userdefined domain
types
create domain person_name char(20) not null
n Types and domains are similar. Domains can have constraints, such
as not null, specified on them.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Domain Constraints
n Domain constraints are the most elementary form of integrity
constraint. They test values inserted in the database, and test queries
to ensure that the comparisons make sense.
n New domains can be created from existing data types
l Example: create domain Dollars numeric(12, 2)
create domain Pounds numeric(12,2)
n We cannot assign or compare a value of type Dollars to a value of
type Pounds.
l However, we can convert type as below
(cast r.A as Pounds)
(Should also multiply by the dollartopound conversionrate)
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
LargeObject Types
n Large objects (photos, videos, CAD files, etc.) are stored as a large
object:
l blob: binary large object object is a large collection of
uninterpreted binary data (whose interpretation is left to an
application outside of the database system)
l clob: character large object object is a large collection of
character data
l When a query returns a large object, a pointer is returned rather
than the large object itself.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Integrity Constraints
n Integrity constraints guard against accidental damage to the
database, by ensuring that authorized changes to the
database do not result in a loss of data consistency.
l A checking account must have a balance greater than
$10,000.00
l A salary of a bank employee must be at least $4.00 an
hour
l A customer must have a (nonnull) phone number
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Constraints on a Single Relation
n not null
n primary key
n unique
n check (P ), where P is a predicate
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Not Null Constraint
n Declare branch_name for branch is not null
branch_name char(15) not null
n Declare the domain Dollars to be not null
create domain Dollars numeric(12,2) not null
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
The Unique Constraint
n unique ( A1, A2, , Am)
n The unique specification states that the attributes
A1, A2, Am
form a candidate key.
n Candidate keys are permitted to be null (in contrast to primary keys).
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
The check clause
n check (P ), where P is a predicate
Example: Declare branch_name as the primary key for
branch and ensure that the values of assets are non
negative.
create table branch
(branch_name char(15),
branch_city char(30),
assets integer,
primary key (branch_name),
check (assets >= 0))
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
The check clause (Cont.)
n The check clause in SQL92 permits domains to be restricted:
l Use check clause to ensure that an hourly_wage domain allows
only values greater than a specified value.
create domain hourly_wage numeric(5,2)
constraint value_test check(value > = 4.00)
l The domain has a constraint that ensures that the hourly_wage is
greater than 4.00
l The clause constraint value_test is optional; useful to indicate
which constraint an update violated.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Referential Integrity
n Ensures that a value that appears in one relation for a given set of
attributes also appears for a certain set of attributes in another relation.
l Example: If “Perryridge” is a branch name appearing in one of the
tuples in the account relation, then there exists a tuple in the branch
relation for branch “Perryridge”.
n Primary and candidate keys and foreign keys can be specified as part of
the SQL create table statement:
l The primary key clause lists attributes that comprise the primary key.
l The unique key clause lists attributes that comprise a candidate key.
l The foreign key clause lists the attributes that comprise the foreign
key and the name of the relation referenced by the foreign key. By
default, a foreign key references the primary key attributes of the
referenced table.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Referential Integrity in SQL – Example
create table customer
(customer_name char(20),
customer_street char(30),
customer_city char(30),
primary key (customer_name ))
create table branch
(branch_name char(15),
branch_city char(30),
assets numeric(12,2),
primary key (branch_name ))
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Referential Integrity in SQL – Example (Cont.)
create table account
(account_number char(10),
branch_name char(15),
balance integer,
primary key (account_number),
foreign key (branch_name) references branch )
create table depositor
(customer_name char(20),
account_number char(10),
primary key (customer_name, account_number),
foreign key (account_number ) references account,
foreign key (customer_name ) references customer )
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Assertions
n An assertion is a predicate expressing a condition that we wish the
database always to satisfy.
n An assertion in SQL takes the form
create assertion check
n When an assertion is made, the system tests it for validity, and tests it
again on every update that may violate the assertion
l This testing may introduce a significant amount of overhead;
hence assertions should be used with great care.
n Asserting
for all X, P(X)
is achieved in a roundabout fashion using
not exists X such that not P(X)
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Assertion Example
n Every loan has at least one borrower who maintains an account with a
minimum balance or $1000.00
create assertion balance_constraint check
(not exists (
select *
from loan
where not exists (
select *
from borrower, depositor, account
where loan.loan_number = borrower.loan_number
and borrower.customer_name = depositor.customer_name
and depositor.account_number = account.account_number
and account.balance >= 1000)))
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Assertion Example
n The sum of all loan amounts for each branch must be less than the
sum of all account balances at the branch.
create assertion sum_constraint check
(not exists (select *
from branch
where (select sum(amount )
from loan
where loan.branch_name =
branch.branch_name )
>= (select sum (amount )
from account
where loan.branch_name =
branch.branch_name )))
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Authorization
Forms of authorization on parts of the database:
n Read allows reading, but not modification of data.
n Insert allows insertion of new data, but not modification of existing data.
n Update allows modification, but not deletion of data.
n Delete allows deletion of data.
Forms of authorization to modify the database schema (covered in Chapter 8):
n Index allows creation and deletion of indices.
n Resources allows creation of new relations.
n Alteration allows addition or deletion of attributes in a relation.
n Drop allows deletion of relations.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Authorization Specification in SQL
n The grant statement is used to confer authorization
grant
on to
n is:
l a userid
l public, which allows all valid users the privilege granted
l A role (more on this in Chapter 8)
n Granting a privilege on a view does not imply granting any privileges
on the underlying relations.
n The grantor of the privilege must already hold the privilege on the
specified item (or be the database administrator).
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Privileges in SQL
n select: allows read access to relation,or the ability to query using
the view
l Example: grant users U1, U2, and U3 select authorization on
the branch relation:
grant select on branch to U1, U2, U3
n insert: the ability to insert tuples
n update: the ability to update using the SQL update statement
n delete: the ability to delete tuples.
n all privileges: used as a short form for all the allowable privileges
n more in Chapter 8
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Revoking Authorization in SQL
n The revoke statement is used to revoke authorization.
revoke
on from
n Example:
revoke select on branch from U1, U2, U3
n may be all to revoke all privileges the revokee may
hold.
n If includes public, all users lose the privilege except
those granted it explicitly.
n If the same privilege was granted twice to the same user by different
grantees, the user may retain the privilege after the revocation.
n All privileges that depend on the privilege being revoked are also
revoked.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Embedded SQL
n The SQL standard defines embeddings of SQL in a variety of
programming languages such as C, Java, and Cobol.
n A language to which SQL queries are embedded is referred to as a host
language, and the SQL structures permitted in the host language
comprise embedded SQL.
n The basic form of these languages follows that of the System R
embedding of SQL into PL/I.
n EXEC SQL statement is used to identify embedded SQL request to the
preprocessor
EXEC SQL END_EXEC
Note: this varies by language (for example, the Java embedding uses
# SQL { . }; )
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Example Query
n Specify the query in SQL and declare a cursor for it
EXEC SQL
declare c cursor for
select depositor.customer_name, customer_city
from depositor, customer, account
where depositor.customer_name = customer.customer_name
and depositor account_number = account.account_number
and account.balance > :amount
END_EXEC
n From within a host language, find the names and cities of
customers with more than the variable amount dollars in some
account.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Embedded SQL (Cont.)
n The open statement causes the query to be evaluated
EXEC SQL open c END_EXEC
n The fetch statement causes the values of one tuple in the query result
to be placed on host language variables.
EXEC SQL fetch c into :cn, :cc END_EXEC
Repeated calls to fetch get successive tuples in the query result
n A variable called SQLSTATE in the SQL communication area
(SQLCA) gets set to ‘02000’ to indicate no more data is available
n The close statement causes the database system to delete the
temporary relation that holds the result of the query.
EXEC SQL close c END_EXEC
Note: above details vary with language. For example, the Java
embedding defines Java iterators to step through result tuples.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Updates Through Cursors
n Can update tuples fetched by cursor by declaring that the cursor is for
update
declare c cursor for
select *
from account
where branch_name = ‘Perryridge’
for update
n To update tuple at the current location of cursor c
update account
set balance = balance + 100
where current of c
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Dynamic SQL
n Allows programs to construct and submit SQL queries at run time.
n Example of the use of dynamic SQL from within a C program.
char * sqlprog = “update account
set balance = balance * 1.05
where account_number = ?”
EXEC SQL prepare dynprog from :sqlprog;
char account [10] = “A101”;
EXEC SQL execute dynprog using :account;
n The dynamic SQL program contains a ?, which is a place holder for a
value that is provided when the SQL program is executed.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
ODBC and JDBC
n API (applicationprogram interface) for a program to interact with a
database server
n Application makes calls to
l Connect with the database server
l Send SQL commands to the database server
l Fetch tuples of result onebyone into program variables
n ODBC (Open Database Connectivity) works with C, C++, C#, and
Visual Basic
n JDBC (Java Database Connectivity) works with Java
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
ODBC
n Open DataBase Connectivity(ODBC) standard
l standard for application program to communicate with a database
server.
l application program interface (API) to
open a connection with a database,
send queries and updates,
get back results.
n Applications such as GUI, spreadsheets, etc. can use ODBC
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
ODBC (Cont.)
n Each database system supporting ODBC provides a "driver" library that
must be linked with the client program.
n When client program makes an ODBC API call, the code in the library
communicates with the server to carry out the requested action, and
fetch results.
n ODBC program first allocates an SQL environment, then a database
connection handle.
n Opens database connection using SQLConnect(). Parameters for
SQLConnect:
l connection handle,
l the server to which to connect
l the user identifier,
l password
n Must also specify types of arguments:
l SQL_NTS denotes previous argument is a nullterminated string.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
ODBC Code
n int ODBCexample()
{
RETCODE error;
HENV env; /* environment */
HDBC conn; /* database connection */
SQLAllocEnv(&env);
SQLAllocConnect(env, &conn);
SQLConnect(conn, "aura.belllabs.com", SQL_NTS, "avi", SQL_NTS,
"avipasswd", SQL_NTS);
{ . Do actual work }
SQLDisconnect(conn);
SQLFreeConnect(conn);
SQLFreeEnv(env);
}
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
ODBC Code (Cont.)
n Program sends SQL commands to the database by using SQLExecDirect
n Result tuples are fetched using SQLFetch()
n SQLBindCol() binds C language variables to attributes of the query result
l When a tuple is fetched, its attribute values are automatically stored in
corresponding C variables.
l Arguments to SQLBindCol()
ODBC stmt variable, attribute position in query result
The type conversion from SQL to C.
The address of the variable.
For variablelength types like character arrays,
– The maximum length of the variable
– Location to store actual length when a tuple is fetched.
– Note: A negative value returned for the length field indicates null value
n Good programming requires checking results of every function call for
errors; we have omitted most checks for brevity.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
ODBC Code (Cont.)
n Main body of program
char branchname[80];
float balance;
int lenOut1, lenOut2;
HSTMT stmt;
SQLAllocStmt(conn, &stmt);
char * sqlquery = "select branch_name, sum (balance)
from account
group by branch_name";
error = SQLExecDirect(stmt, sqlquery, SQL_NTS);
if (error == SQL_SUCCESS) {
SQLBindCol(stmt, 1, SQL_C_CHAR, branchname , 80,
&lenOut1);
SQLBindCol(stmt, 2, SQL_C_FLOAT, &balance, 0 ,
&lenOut2);
while (SQLFetch(stmt) >= SQL_SUCCESS) {
printf (" %s %g\n", branchname, balance);
}
}
SQLFreeStmt(stmt, SQL_DROP);
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
More ODBC Features
n Prepared Statement
l SQL statement prepared: compiled at the database
l Can have placeholders: E.g. insert into account values(?,?,?)
l Repeatedly executed with actual values for the placeholders
n Metadata features
l finding all the relations in the database and
l finding the names and types of columns of a query result or a relation in
the database.
n By default, each SQL statement is treated as a separate transaction that is
committed automatically.
l Can turn off automatic commit on a connection
SQLSetConnectOption(conn, SQL_AUTOCOMMIT, 0)}
l transactions must then be committed or rolled back explicitly by
SQLTransact(conn, SQL_COMMIT) or
SQLTransact(conn, SQL_ROLLBACK)
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
ODBC Conformance Levels
n Conformance levels specify subsets of the functionality defined by the
standard.
l Core
l Level 1 requires support for metadata querying
l Level 2 requires ability to send and retrieve arrays of parameter
values and more detailed catalog information.
n SQL Call Level Interface (CLI) standard similar to ODBC interface, but
with some minor differences.
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
JDBC
n JDBC is a Java API for communicating with database systems
supporting SQL
n JDBC supports a variety of features for querying and updating data, and
for retrieving query results
n JDBC also supports metadata retrieval, such as querying about relations
present in the database and the names and types of relation attributes
n Model for communicating with the database:
l Open a connection
l Create a “statement” object
l Execute queries using the Statement object to send queries and
fetch results
l Exception mechanism to handle errors
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
JDBC Code
public static void JDBCexample(String dbid, String userid, String passwd)
{
try {
Class.forName ("oracle.jdbc.driver.OracleDriver");
Connection conn = DriverManager.getConnection(
"jdbc:oracle:thin:@aura.belllabs.com:2000:bankdb", userid, passwd);
Statement stmt = conn.createStatement();
Do Actual Work .
stmt.close();
conn.close();
}
catch (SQLException sqle) {
System.out.println("SQLException : " + sqle);
}
}
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
JDBC Code (Cont.)
n Update to database
try {
stmt.executeUpdate( "insert into account values
('A9732', 'Perryridge', 1200)");
} catch (SQLException sqle) {
System.out.println("Could not insert tuple. " + sqle);
}
n Execute query and fetch and print results
ResultSet rset = stmt.executeQuery( "select branch_name,
avg(balance)
from account
group by branch_name");
while (rset.next()) {
System.out.println(
rset.getString("branch_name") + " " + rset.getFloat(2));
}
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
JDBC Code Details
n Getting result fields:
l rs.getString(“branchname”) and rs.getString(1) equivalent if
branchname is the first argument of select result.
n Dealing with Null values
int a = rs.getInt(“a”);
if (rs.wasNull()) Systems.out.println(“Got null value”);
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Procedural Extensions and Stored Procedures
n SQL provides a module language
l Permits definition of procedures in SQL, with ifthenelse statements,
for and while loops, etc.
l more in Chapter 9
n Stored Procedures
l Can store procedures in the database
l then execute them using the call statement
l permit external applications to operate on the database without
knowing about internal details
n These features are covered in Chapter 9 (Object Relational Databases)
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Functions and Procedures
n SQL:1999 supports functions and procedures
l Functions/procedures can be written in SQL itself, or in an external
programming language
l Functions are particularly useful with specialized data types such as
images and geometric objects
Example: functions to check if polygons overlap, or to compare
images for similarity
l Some database systems support tablevalued functions, which
can return a relation as a result
n SQL:1999 also supports a rich set of imperative constructs, including
l Loops, ifthenelse, assignment
n Many databases have proprietary procedural extensions to SQL that
differ from SQL:1999
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
SQL Functions
n Define a function that, given the name of a customer, returns the count
of the number of accounts owned by the customer.
create function account_count (customer_name varchar(20))
returns integer
begin
declare a_count integer;
select count (* ) into a_count
from depositor
where depositor.customer_name = customer_name
return a_count;
end
n Find the name and address of each customer that has more than one
account.
select customer_name, customer_street, customer_city
from customer
where account_count (customer_name ) > 1
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Table Functions
n SQL:2003 added functions that return a relation as a result
n Example: Return all accounts owned by a given customer
create function accounts_of (customer_name char(20)
returns table ( account_number char(10),
branch_name char(15)
balance numeric(12,2))
return table
(select account_number, branch_name, balance
from account A
where exists (
select *
from depositor D
where D.customer_name = accounts_of.customer_name
and D.account_number = A.account_number ))
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Table Functions (cont’d)
n Usage
select *
from table (accounts_of (‘Smith’))
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
SQL Procedures
n The author_count function could instead be written as procedure:
create procedure account_count_proc (in title varchar(20),
out a_count integer)
begin
select count(author) into a_count
from depositor
where depositor.customer_name = account_count_proc.customer_name
end
n Procedures can be invoked either from an SQL procedure or from
embedded SQL, using the call statement.
declare a_count integer;
call account_count_proc( ‘Smith’, a_count);
Procedures and functions can be invoked also from dynamic SQL
n SQL:1999 allows more than one function/procedure of the same name
(called name overloading), as long as the number of
arguments differ, or at least the types of the arguments differ
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Procedural Constructs
n Compound statement: begin end,
l May contain multiple SQL statements between begin and end.
l Local variables can be declared within a compound statements
n While and repeat statements:
declare n integer default 0;
while n < 10 do
set n = n + 1
end while
repeat
set n = n – 1
until n = 0
end repeat
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Procedural Constructs (Cont.)
n For loop
l Permits iteration over all results of a query
l Example: find total of all balances at the Perryridge branch
declare n integer default 0;
for r as
select balance from account
where branch_name = ‘Perryridge’
do
set n = n + r.balance
end for
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Procedural Constructs (cont.)
n Conditional statements (ifthenelse)
E.g. To find sum of balances for each of three categories of accounts
(with balance =1000 and = 5000)
if r.balance < 1000
then set l = l + r.balance
elseif r.balance < 5000
then set m = m + r.balance
else set h = h + r.balance
end if
n SQL:1999 also supports a case statement similar to C case statement
n Signaling of exception conditions, and declaring handlers for exceptions
declare out_of_stock condition
declare exit handler for out_of_stock
begin
.. signal outofstock
end
l The handler here is exit causes enclosing begin..end to be exited
l Other actions possible on exception
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
External Language Functions/Procedures
n SQL:1999 permits the use of functions and procedures written in other
languages such as C or C++
n Declaring external language procedures and functions
create procedure account_count_proc(in customer_name varchar(20),
out count integer)
language C
external name ’ /usr/avi/bin/account_count_proc’
create function account_count(customer_name varchar(20))
returns integer
language C
external name ‘/usr/avi/bin/author_count’
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
External Language Routines (Cont.)
n Benefits of external language functions/procedures:
l more efficient for many operations, and more expressive power
n Drawbacks
l Code to implement function may need to be loaded into database
system and executed in the database system’s address space
risk of accidental corruption of database structures
security risk, allowing users access to unauthorized data
l There are alternatives, which give good security at the cost of
potentially worse performance
l Direct execution in the database system’s space is used when
efficiency is more important than security
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Security with External Language Routines
n To deal with security problems
l Use sandbox techniques
that is use a safe language like Java, which cannot be used to
access/damage other parts of the database code
l Or, run external language functions/procedures in a separate
process, with no access to the database process’ memory
Parameters and results communicated via interprocess
communication
n Both have performance overheads
n Many database systems support both above approaches as well as
direct executing in database system address space
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Recursion in SQL
n SQL:1999 permits recursive view definition
n Example: find all employeemanager pairs, where the employee
reports to the manager directly or indirectly (that is manager’s
manager, manager’s manager’s manager, etc.)
with recursive empl (employee_name, manager_name ) as (
select employee_name, manager_name
from manager
union
select manager.employee_name, empl.manager_name
from manager, empl
where manager.manager_name = empl.employe_name)
select *
from empl
This example view, empl, is called the transitive closure of the
manager relation
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
The Power of Recursion
n Recursive views make it possible to write queries, such as transitive
closure queries, that cannot be written without recursion or iteration.
l Intuition: Without recursion, a nonrecursive noniterative program
can perform only a fixed number of joins of manager with itself
This can give only a fixed number of levels of managers
Given a program we can construct a database with a greater
number of levels of managers on which the program will not work
n Computing transitive closure
l The next slide shows a manager relation
l Each step of the iterative process constructs an extended version of
empl from its recursive definition.
l The final result is called the fixed point of the recursive view
definition.
n Recursive views are required to be monotonic. That is, if we add tuples
to manger the view contains all of the tuples it contained before, plus
possibly more
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Example of FixedPoint Computation
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Advanced SQL Features**
n Create a table with the same schema as an existing table:
create table temp_account like account
n SQL:2003 allows subqueries to occur anywhere a value is required
provided the subquery returns only one value. This applies to updates as
well
n SQL:2003 allows subqueries in the from clause to access attributes of
other relations in the from clause using the lateral construct:
select C.customer_name, num_accounts
from customer C,
lateral (select count(*)
from account A
where A.customer_name = C.customer_name )
as this_customer (num_accounts )
©Silberschatz, Korth and Sudarshan4.Database System Concepts, 5th Ed., July 2005
Advanced SQL Features (cont’d)
n Merge construct allows batch processing of updates.
n Example: relation funds_received (account_number, amount ) has
batch of deposits to be added to the proper account in the account
relation
merge into account as A
using (select *
from funds_received as F )
on (A.account_number = F.account_number )
when matched then
update set balance = balance + F.amount
Database System Concepts, 5th Ed.
©Silberschatz, Korth and Sudarshan
See www.dbbook.com for conditions on reuse
End of Chapter
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