Chapter 4: SQL
n Basic Structure
n Set Operations
n Aggregate Functions
n Null Values
n Nested Subqueries
n Derived Relations
n Views
n Modification of the Database
n Joined Relations
n Data Definition Language
n Embedded SQL, ODBC and JDBC
]
Schema Used in Examples
Basic Structure
n SQL is based on set and relational operations with certain modifications and enhancements
n A typical SQL query has the form:
select A1, A2, ..., An
from r1, r2, ..., rm
where P
H Ais represent attributes
H ris represent relations
H P is a predicate.
n This query is equivalent to the relational algebra expression.
A1, A2, ..., An(P (r1 x r2 x ... x rm))
n The result of an SQL query is a relation.
The select Clause
n The select clause list the attributes desired in the result of a query
H corresponds to the projection operation of the relational algebra
n E.g. find the names of all branches in the loan relation
select branch-name
from loan
n In the "pure" relational algebra syntax, the query would be:
branch-name(loan)
n NOTE: SQL does not permit the '-' character in names,
H Use, e.g., branch_name instead of branch-name in a real implementation.
H We use '-' since it looks nicer!
n NOTE: SQL names are case insensitive, i.e. you can use capital or small letters.
H You may wish to use upper case where-ever we use bold font.
n SQL allows duplicates in relations as well as in query results.
n To force the elimination of duplicates, insert the keyword distinct after select.
n Find the names of all branches in the loan relations, and remove duplicates
select distinct branch-name
from loan
n The keyword all specifies that duplicates not be removed.
select all branch-name
from loan
n An asterisk in the select clause denotes "all attributes"
select *
from loan
n The select clause can contain arithmetic expressions involving the operation, +, –, , and /, and operating on constants or attributes of tuples.
n The query:
select loan-number, branch-name, amount 100
from loan
would return a relation which is the same as the loan relations, except that the attribute amount is multiplied by 100.
The where Clause
n The where clause specifies conditions that the result must satisfy
H corresponds to the selection predicate of the relational algebra.
n To find all loan number for loans made at the Perryridge branch with loan amounts greater than $1200.
select loan-number
from loan
where branch-name = 'Perryridge' and amount > 1200
n Comparison results can be combined using the logical connectives and, or, and not.
n Comparisons can be applied to results of arithmetic expressions.
n SQL includes a between comparison operator
n E.g. Find the loan number of those loans with loan amounts between $90,000 and $100,000 (that is, $90,000 and $100,000)
select loan-number
from loan
where amount between 90000 and 100000
The from Clause
n The from clause lists the relations involved in the query
H corresponds to the Cartesian product operation of the relational algebra.
n Find the Cartesian product borrower x loan select *
from borrower, loan
nFind the name, loan number and loan amount of all customers
having a loan at the Perryridge branch.
select customer-name, borrower.loan-number, amount
from borrower, loan
where borrower.loan-number = loan.loan-number and
branch-name = 'Perryridge'
The Rename Operation
n The SQL allows renaming relations and attributes using the as clause:
old-name as new-name
n Find the name, loan number and loan amount of all customers; rename the column name loan-number as loan-id.
select customer-name, borrower.loan-number as loan-id, amount
from borrower, loan
where borrower.loan-number = loan.loan-number
Tuple Variables
n Tuple variables are defined in the from clause via the use of the as clause.
n Find the customer names and their loan numbers for all customers having a loan at some branch.
select customer-name, T.loan-number, S.amount
from borrower as T, loan as S
where T.loan-number = S.loan-number
nFind the names of all branches that have greater assets than
some branch located in Brooklyn.
select distinct T.branch-name
from branch as T, branch as S
where T.assets > S.assets and S.branch-city = 'Brooklyn'
String Operations
n SQL includes a string-matching operator for comparisons on character strings. Patterns are described using two special characters:
H percent (%). The % character matches any substring.
H underscore (_). The _ character matches any character.
n Find the names of all customers whose street includes the substring "Main".
select customer-name
from customer
where customer-street like '%Main%'
n Match the name "Main%"
like 'Main\%' escape '\'
n SQL supports a variety of string operations such as
H concatenation (using "||")
H converting from upper to lower case (and vice versa)
H finding string length, extracting substrings, etc.
Ordering the Display of Tuples
n List in alphabetic order the names of all customers having a loan in Perryridge branch
select distinct customer-name
from borrower, loan
where borrower loan-number - loan.loan-number and
branch-name = 'Perryridge'
order by customer-name
n We may specify desc for descending order or asc for ascending order, for each attribute; ascending order is the default.
H E.g. order by customer-name desc
Duplicates
n In relations with duplicates, SQL can define how many copies of tuples appear in the result.
n Multiset versions of some of the relational algebra operators – given multiset relations r1 and r2:
1. sq (r1): If there are c1 copies of tuple t1 in r1, and t1 satisfies selections sq,, then there are c1 copies of t1 in sq (r1).
2. PA(r): For each copy of tuple t1 in r1, there is a copy of tuple PA(t1) in PA(r1) where PA(t1) denotes the projection of the single tuple t1.
3. r1 x r2 : If there are c1 copies of tuple t1 in r1 and c2 copies of tuple t2 in r2, there are c1 x c2 copies of the tuple t1. t2 in r1 x r2
n Example: Suppose multiset relations r1 (A, B) and r2 (C) are as follows:
r1 = {(1, a) (2,a)} r2 = {(2), (3), (3)}
n Then PB(r1) would be {(a), (a)}, while PB(r1) x r2 would be
{(a,2), (a,2), (a,3), (a,3), (a,3), (a,3)}
n SQL duplicate semantics:
select A1,, A2, ..., An
from r1, r2, ..., rm
where P
is equivalent to the multiset version of the expression:
P A1,, A2, ..., An(sP (r1 x r2 x ... x rm))
Set Operations
n The set operations union, intersect, and except operate on relations and correspond to the relational algebra operations È, Ç, -.
n Each of the above operations automatically eliminates duplicates; to retain all duplicates use the corresponding multiset versions union all, intersect all and except all.
Suppose a tuple occurs m times in r and n times in s, then, it occurs:
H m + n times in r union all s
H min(m,n) times in r intersect all s
H max(0, m – n) times in r except all s
Set Operations
n Find all customers who have a loan, an account, or both:
(select customer-name from depositor)
union
(select customer-name from borrower)
nFind all customers who have both a loan and an account.
(select customer-name from depositor)
intersect
(select customer-name from borrower)
nFind all customers who have an account but no loan.
(select customer-name from depositor)
except
(select customer-name from borrower)
Aggregate Functions
n These functions operate on the multiset of values of a column of a relation, and return a value
avg: average value
min: minimum value
max: maximum value
sum: sum of values
count: number of values
n Find the average account balance at the Perryridge branch.
select avg (balance)
from account
where branch-name = 'Perryridge'
nFind the number of tuples in the customer relation.
select count (*)
from customer
nFind the number of depositors in the bank.
select count (distinct customer-name)
from depositor
Aggregate Functions – Group By
n Find the number of depositors for each branch.
select branch-name, count (distinct customer-name)
from depositor, account
where depositor.account-number = account.account-number
group by branch-name
Note: Attributes in select clause outside of aggregate functions must
appear in group by list
Aggregate Functions – Having Clause
n Find the names of all branches where the average account balance is more than $1,200.
select branch-name, avg (balance)
from account
group by branch-name
having avg (balance) > 1200
Note: predicates in the having clause are applied after the
formation of groups whereas predicates in the where
clause are applied before forming groups
Null Values
n It is possible for tuples to have a null value, denoted by null, for some of their attributes
n null signifies an unknown value or that a value does not exist.
n The predicate is null can be used to check for null values.
H E.g. Find all loan number which appear in the loan relation with null values for amount.
select loan-number
from loan
where amount is null
n The result of any arithmetic expression involving null is null
H E.g. 5 + null returns null
n However, aggregate functions simply ignore nulls
H more on this shortly
Null Values and Three Valued Logic
n Any comparison with null returns unknown
H E.g. 5 < null or null <> null or null = null
n Three-valued logic using the truth value unknown:
H OR: (unknown or true) = true, (unknown or false) = unknown
(unknown or unknown) = unknown
H AND: (true and unknown) = unknown, (false and unknown) = false,
(unknown and unknown) = unknown
H NOT: (not unknown) = unknown
H "P is unknown" evaluates to true if predicate P evaluates to unknown
n Result of where clause predicate is treated as false if it evaluates to unknown
Null Values and Aggregates
n Total all loan amounts
select sum (amount)
from loan
H Above statement ignores null amounts
H result is null if there is no non-null amount, that is the
n All aggregate operations except count(*) ignore tuples with null values on the aggregated attributes.
Nested Subqueries
n SQL provides a mechanism for the nesting of subqueries.
n A subquery is a select-from-where expression that is nested within another query.
n A common use of subqueries is to perform tests for set membership, set comparisons, and set cardinality.
Example Query
n Find all customers who have both an account and a loan at the bank.
select distinct customer-name
from borrower
where customer-name in (select customer-name
from depositor)
Find all customers who have a loan at the bank but do not have
an account at the bank
select distinct customer-name
from borrower
where customer-name not in (select customer-name
from depositor)
Example Query
n Find all customers who have both an account and a loan at the Perryridge branch
select distinct customer-name
from borrower, loan
where borrower.loan-number = loan.loan-number and
branch-name = "Perryridge" and
(branch-name, customer-name) in
(select branch-name, customer-name
from depositor, account
where depositor.account-number =
account.account-number)
nNote: Above query can be written in a much simpler manner. The
formulation above is simply to illustrate SQL features.
(Schema used in this example)
Set Comparison
n Find all branches that have greater assets than some branch located in Brooklyn.
select distinct T.branch-name
from branch as T, branch as S
where T.assets > S.assets and
S.branch-city = 'Brooklyn'
nSame query using > some clause
select branch-name
from branch
where assets > some
(select assets
from branch
where branch-city = 'Brooklyn')
Definition of Some Clause
n F <comp> some r Û $ t Î r s.t. (F <comp> t)
Where <comp> can be: <, £, >, =, ¹
) = false
) = true
) = false |
) = true |
Definition of all Clause
n F <comp> all r Û " t Î r (F <comp> t)
) = false
) = true
) = false
) = false |
) = true |
) = false |
Example Query
n Find the names of all branches that have greater assets than all branches located in Brooklyn.
select branch-name
from branch
where assets > all
(select assets
from branch
where branch-city = 'Brooklyn')
Test for Empty Relations
n The exists construct returns the value true if the argument subquery is nonempty.
n exists r Û r ¹ Ø
n not exists r Û r = Ø
Example Query
n Find all customers who have an account at all branches located in Brooklyn.
select distinct S.customer-name
from depositor as S
where not exists (
(select branch-name
from branch
where branch-city = 'Brooklyn')
except
(select R.branch-name
from depositor as T, account as R
where T.account-number = R.account-number and
S.customer-name = T.customer-name))
n(Schema used in this example)
n Note that X – Y = Ø Û X Í Y
n Note: Cannot write this query using = all and its variants
Test for Absence of Duplicate Tuples
n The unique construct tests whether a subquery has any duplicate tuples in its result.
n Find all customers who have at most one account at the Perryridge branch.
select T.customer-name
from depositor as T
where unique (
select R.customer-name
from account, depositor as R
where T.customer-name = R.customer-name and
R.account-number = account.account-number and
account.branch-name = 'Perryridge')
n (Schema used in this example)
Example Query
n Find all customers who have at least two accounts at the Perryridge branch.
select distinct T.customer-name
from depositor T
where not unique (
select R.customer-name
from account, depositor as R
where T.customer-name = R.customer-name
and R.account-number = account.account-number
and
account.branch-name = 'Perryridge')
n(Schema used in this example)
Views
n Provide a mechanism to hide certain data from the view of certain users. To create a view we use the command:
create view v as <query expression>
where:
H<query expression> is any legal expression
HThe view name is represented by v
Example Queries
n A view consisting of branches and their customers
create view all-customer as
(select branch-name, customer-name
from depositor, account
where depositor.account-number = account.account-number) union
(select branch-name, customer-name
from borrower, loan
where borrower.loan-number = loan.loan-number)
nFind all customers of the Perryridge branch
select customer-name
from all-customer
where branch-name = 'Perryridge'
Derived Relations
n Find the average account balance of those branches where the average account balance is greater than $1200.
select branch-name, avg-balance
from (select branch-name, avg (balance)
from account
group by branch-name)
as result (branch-name, avg-balance)
where avg-balance > 1200
Note that we do not need to use the having clause, since we compute the temporary (view) relation result in the from clause, and the attributes of result can be used directly in the where clause.
With Clause
n With clause allows views to be defined locally to a query, rather than globally. Analogous to procedures in a programming language.
n Find all accounts with the maximum balance
with max-balance(value) as
select max (balance)
from account
select account-number
from account, max-balance
where account.balance = max-balance.value
Complex Query using With Clause
n Find all branches where the total account deposit is greater than the average of the total account deposits at all branches.
with branch-total (branch-name, value) as
select branch-name, sum (balance)
from account
group by branch-name
with branch-total-avg(value) as
select avg (value)
from branch-total
select branch-name
from branch-total, branch-total-avg
where branch-total.value >= branch-total-avg.value
Modification of the Database – Deletion
n Delete all account records at the Perryridge branch
delete from account
where branch-name = 'Perryridge'
n Delete all accounts at every branch located in Needham city.
delete from account
where branch-name in (select branch-name
from branch
where branch-city = 'Needham')
delete from depositor
where account-number in
(select account-number
from branch, account
where branch-city = 'Needham'
and branch.branch-name = account.branch-name)
n (Schema used in this example)
Example Query
n Delete the record of all accounts with balances below the average at the bank.
delete from account
where balance < (select avg (balance)
from account)
HProblem: as we delete tuples from deposit, the average balance
changes
H Solution used in SQL:
1. First, compute avg balance and find all tuples to delete
2. Next, delete all tuples found above (without recomputing avg or
retesting the tuples)
Modification of the Database – Insertion
n Add a new tuple to account
insert into account
values ('A-9732', 'Perryridge',1200)
or equivalently
insert into account (branch-name, balance, account-number)
values ('Perryridge', 1200, 'A-9732')
n Add a new tuple to account with balance set to null
insert into account
values ('A-777','Perryridge', null)
Modification of the Database – Insertion
n Provide as a gift for all loan customers of the Perryridge branch, a $200 savings account. Let the loan number serve as the account number for the new savings account
insert into account
select loan-number, branch-name, 200
from loan
where branch-name = 'Perryridge'
insert into depositor
select customer-name, loan-number
from loan, borrower
where branch-name = 'Perryridge'
and loan.account-number = borrower.account-number
n The select from where statement is fully evaluated before any of its results are inserted into the relation (otherwise queries like
insert into table1 select * from table1
would cause problems
Modification of the Database – Updates
n Increase all accounts with balances over $10,000 by 6%, all other accounts receive 5%.
H Write two update statements:
update account
set balance = balance * 1.06
where balance > 10000
update account
set balance = balance * 1.05
where balance £ 10000
H The order is important
H Can be done better using the case statement
Case Statement for Conditional Updates
n Same query as before: Increase all accounts with balances over $10,000 by 6%, all other accounts receive 5%.
update account
set balance = case
when balance <= 10000 then balance *1.05
else balance * 1.06
end
Update of a View
n Create a view of all loan data in loan relation, hiding the amount attribute
create view branch-loan as
select branch-name, loan-number
from loan
n Add a new tuple to branch-loan
insert into branch-loan
values ('Perryridge', 'L-307')
This insertion must be represented by the insertion of the tuple
('L-307', 'Perryridge', null)
into the loan relation
n Updates on more complex views are difficult or impossible to translate, and hence are disallowed.
n Most SQL implementations allow updates only on simple views (without aggregates) defined on a single relation
Transactions
n A transaction is a sequence of queries and update statements executed as a single unit
H Transactions are started implicitly and terminated by one of
4 commit work: makes all updates of the transaction permanent in the database
4 rollback work: undoes all updates performed by the transaction.
n Motivating example
H Transfer of money from one account to another involves two steps:
4 deduct from one account and credit to another
H If one steps succeeds and the other fails, database is in an inconsistent state
H Therefore, either both steps should succeed or neither should
n If any step of a transaction fails, all work done by the transaction can be undone by rollback work.
n Rollback of incomplete transactions is done automatically, in case of system failures
n In most database systems, each SQL statement that executes successfully is automatically committed.
H Each transaction would then consist of only a single statement
H Automatic commit can usually be turned off, allowing multi-statement transactions, but how to do so depends on the database system
H Another option in SQL:1999: enclose statements within
begin atomic
…
end
Joined Relations
n Join operations take two relations and return as a result another relation.
n These additional operations are typically used as subquery expressions in the from clause
n Join condition – defines which tuples in the two relations match, and what attributes are present in the result of the join.
n Join type – defines how tuples in each relation that do not match any tuple in the other relation (based on the join condition) are treated.
Joined Relations – Datasets for Examples
n Relation loan
nRelation borrower
nNote: borrower information missing for L-260 and loan information missing for L-155
Joined Relations – Examples
n loan inner join borrower on
loan.loan-number = borrower.loan-number
nloan left outer join borrower on
loan.loan-number = borrower.loan-number
Joined Relations – Examples
n loan natural inner join borrower
nloan natural right outer join borrower
Joined Relations – Examples
n loan full outer join borrower using (loan-number)
nFind all customers who have either an account or a loan (but not both) at the bank.
select customer-name
from (depositor natural full outer join borrower)
where account-number is null or loan-number is null
Data Definition Language (DDL)
Allows the specification of not only a set of relations but also information about each relation, including:
n The schema for each relation.
n The domain of values associated with each attribute.
n Integrity constraints
n The set of indices to be maintained for each relations.
n Security and authorization information for each relation.
n The physical storage structure of each relation on disk.
Domain Types in SQL
n char(n). Fixed length character string, with user-specified length n.
n varchar(n). Variable length character strings, with user-specified maximum length n.
n int. Integer (a finite subset of the integers that is machine-dependent).
n smallint. Small integer (a machine-dependent subset of the integer domain type).
n numeric(p,d). Fixed point number, with user-specified precision of p digits, with n digits to the right of decimal point.
n real, double precision. Floating point and double-precision floating point numbers, with machine-dependent precision.
n float(n). Floating point number, with user-specified precision of at least n digits.
n Null values are allowed in all the domain types. Declaring an attribute to be not null prohibits null values for that attribute.
n create domain construct in SQL-92 creates user-defined domain types
create domain person-name char(20) not null
Date/Time Types in SQL
date. Dates, containing a (4 digit) year, month and date
H E.g. date '2001-7-27'
n time. Time of day, in hours, minutes and seconds.
H E.g. time '09:00:30' time '09:00:30.75'
n timestamp: date plus time of day
H E.g. timestamp '2001-7-27 09:00:30.75'
n Interval: period of time
H E.g. Interval '1' day
H Subtracting a date/time/timestamp value from another gives an interval value
H Interval values can be added to date/time/timestamp values
n Can extract values of individual fields from date/time/timestamp
H E.g. extract (year from r.starttime)
n Can cast string types to date/time/timestamp
H E.g. cast <string-valued-expression> as date
Create Table Construct
n An SQL relation is defined using the create table command:
create table r (A1 D1, A2 D2, ..., An Dn,
(integrity-constraint1),
...,
(integrity-constraintk))
H r is the name of the relation
H each Ai is an attribute name in the schema of relation r
H Di is the data type of values in the domain of attribute Ai
n Example:
create table branch
(branch-name char(15) not null,
branch-city char(30),
assets integer)
Integrity Constraints in Create Table
n not null
n primary key (A1, ..., An)
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))
primary key declaration on an attribute automatically ensures not null in SQL-92 onwards, needs to be explicitly stated in SQL-89
Drop and Alter Table Constructs
n The drop table command deletes all information about the dropped relation from the database.
n The alter table command is used to add attributes to an existing relation.
alter table r add A D
where A is the name of the attribute to be added to relation r and D is the domain of A.
H All tuples in the relation are assigned null as the value for the new attribute.
n The alter table command can also be used to drop attributes of a relation
alter table r drop A
where A is the name of an attribute of relation r
H Dropping of attributes not supported by many databases
Embedded SQL
n The SQL standard defines embeddings of SQL in a variety of programming languages such as Pascal, PL/I, Fortran, C, 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 <embedded SQL statement > END-EXEC
Note: this varies by language. E.g. the Java embedding uses
# SQL { …. } ;
Example Query
From within a host language, find the names and cities of customers with more than the variable amount dollars in some account.
n Specify the query in SQL and declare a cursor for it
EXEC SQL
declare c cursor for
select 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 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. E.g. the Java embedding defines Java iterators to step through result tuples
.
Updates Through Cursors
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] = "A-101";
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.
ODBC
n Open DataBase Connectivity(ODBC) standard
H standard for application program to communicate with a database server.
H application program interface (API) to
4 open a connection with a database,
4 send queries and updates,
4 get back results.
n Applications such as GUI, spreadsheets, etc. can use ODBC
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:
H connection handle,
H the server to which to connect
H the user identifier,
H password
n Must also specify types of arguments:
H SQL_NTS denotes previous argument is a null-terminated string.
ODBC Code
n int ODBCexample()
{
RETCODE error;
HENV env; /* environment */
HDBC conn; /* database connection */
SQLAllocEnv(&env);
SQLAllocConnect(env, &conn);
SQLConnect(conn, "aura.bell-labs.com", SQL_NTS, "avi", SQL_NTS, "avipasswd", SQL_NTS);
{ …. Do actual work … }
SQLDisconnect(conn);
SQLFreeConnect(conn);
SQLFreeEnv(env);
}
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
4 When a tuple is fetched, its attribute values are automatically stored in corresponding C variables.
4 Arguments to SQLBindCol()
– ODBC stmt variable, attribute position in query result
– The type conversion from SQL to C.
– The address of the variable.
– For variable-length 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.
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);
More ODBC Features
n Prepared Statement
H SQL statement prepared: compiled at the database
H Can have placeholders: E.g. insert into account values(?,?,?)
H Repeatedly executed with actual values for the placeholders
n Metadata features
H finding all the relations in the database and
H 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.
H Can turn off automatic commit on a connection
4 SQLSetConnectOption(conn, SQL_AUTOCOMMIT, 0)}
H transactions must then be committed or rolled back explicitly by
4 SQLTransact(conn, SQL_COMMIT) or
4 SQLTransact(conn, SQL_ROLLBACK)
ODBC Conformance Levels
n Conformance levels specify subsets of the functionality defined by the standard.
H Core
H Level 1 requires support for metadata querying
H 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.
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:
H Open a connection
H Create a "statement" object
H Execute queries using the Statement object to send queries and fetch results
H Exception mechanism to handle errors
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.bell-labs.com:2000:bankdb", userid, passwd);
Statement stmt = conn.createStatement();
… Do Actual Work ….
stmt.close();
conn.close();
}
catch (SQLException sqle) {
System.out.println("SQLException : " + sqle);
}
}
n Update to database
try {
stmt.executeUpdate( "insert into account values
('A-9732', '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));
}
JDBC Code Details
n Getting result fields:
H 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");
Prepared Statement
n Prepared statement allows queries to be compiled and executed multiple times with different arguments
PreparedStatement pStmt = conn.prepareStatement(
"insert into account values(?,?,?)"); pStmt.setString(1, "A-9732");
pStmt.setString(2, "Perryridge");
pStmt.setInt(3, 1200);
pStmt.executeUpdate();
pStmt.setString(1, "A-9733");
pStmt.executeUpdate();
n Beware: If value to be stored in database contains a single quote or other special character, prepared statements work fine, but creating a query string and executing it directly would result in a syntax error!
Other SQL Features
n SQL sessions
H client connects to an SQL server, establishing a session
H executes a series of statements
H disconnects the session
H can commit or rollback the work carried out in the session
n An SQL environment contains several components, including a user identifier, and a schema, which identifies which of several schemas a session is using.
Schemas, Catalogs, and Environments
n Three-level hierarchy for naming relations.
H Database contains multiple catalogs
H each catalog can contain multiple schemas
H SQL objects such as relations and views are contained within a schema
n e.g. catalog5.bank-schema.account
n Each user has a default catalog and schema, and the combination is unique to the user.
n Default catalog and schema are set up for a connection
n Catalog and schema can be omitted, defaults are assumed
n Multiple versions of an application (e.g. production and test) can run under separate schemas
Procedural Extensions and Stored Procedures
n SQL provides a module language
H permits definition of procedures in SQL, with if-then-else statements, for and while loops, etc.
H more in Chapter 9
n Stored Procedures
H Can store procedures in the database
H then execute them using the call statement
H permit external applications to operate on the database without knowing about internal details
n These features are covered in Chapter 9 (Object Relational Databases)
Extra Material on JDBC and Application Architectures
Transactions in JDBC
n As with ODBC, each statement gets committed automatically in JDBC
n To turn off auto commit use
conn . setAutoCommit (false);
n To commit or abort transactions use
conn .commit() or conn .rollback()
n To turn auto commit on again, use
conn . setAutoCommit (true);
Procedure and Function Calls in JDBC
n JDBC provides a class CallableStatement which allows SQL stored procedures/functions to be invoked.
CallableStatement cs1 = conn.prepareCall( "{call proc (?,?)}" ) ;
CallableStatement cs2 = conn.prepareCall( "{? = call func (?,?)}" );
Result Set MetaData
n The class ResultSetMetaData provides information about all the columns of the ResultSet.
n Instance of this class is obtained by getMetaData( ) function of ResultSet.
n Provides Functions for getting number of columns, column name, type, precision, scale, table from which the column is derived etc.
ResultSetMetaData rsmd = rs.getMetaData ( );
for ( int i = 1; i <= rsmd.getColumnCount( ); i++ ) {
String name = rsmd.getColumnName(i);
String typeName = rsmd.getColumnTypeName(i);
}
Database Meta Data
n The class DatabaseMetaData provides information about database relations
n Has functions for getting all tables, all columns of the table, primary keys etc.
n E.g. to print column names and types of a relation
DatabaseMetaData dbmd = conn.getMetaData( );
ResultSet rs = dbmd.getColumns( null, "BANK-DB", "account", "%" );
//Arguments: catalog, schema-pattern, table-pattern, column-pattern
// Returns: 1 row for each column, with several attributes such as
// COLUMN_NAME, TYPE_NAME, etc.
while ( rs.next( ) ) {
System.out.println( rs.getString("COLUMN_NAME") ,
rs.getString("TYPE_NAME");
}
n There are also functions for getting information such as
H Foreign key references in the schema
H Database limits like maximum row size, maximum no. of connections, etc
Application Architectures
n Applications can be built using one of two architectures
H Two tier model
4 Application program running at user site directly uses JDBC/ODBC to communicate with the database
H Three tier model
4 Users/programs running at user sites communicate with an application server. The application server in turn communicates with the database
Two-tier Model
n E.g. Java code runs at client site and uses JDBC to communicate with the backend server
n Benefits:
H flexible, need not be restricted to predefined queries
n Problems:
H Security: passwords available at client site, all database operation possible
H More code shipped to client
H Not appropriate across organizations, or in large ones like universities
Three Tier Model
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