This document provides an overview of SQL and embedded SQL concepts. It discusses scalar subqueries, embedded SQL programming, transactions, dynamic SQL, and JDBC. Scalar subqueries return single values that can be used in expressions or output clauses. Embedded SQL allows embedding SQL statements in programming languages for connectivity. Transactions define units of work that can be committed or rolled back. Dynamic SQL builds SQL statements dynamically at runtime using strings, while JDBC is the Java database connectivity API.

1. SQL
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Fall 2001 Database Systems 1
Scalar subqueries
• A scalar subquery is any expression that produces a
single value (single tuple/ single column)
• Examples:
SELECT max(amount) FROM bids
SELECT count(*) FROM items
• Scalar subqueries can be used as if they were
“constants”
– in expressions in the WHERE clause, and
– to output values in the SELECT clause
Fall 2001 Database Systems 2
Scalar subqueries
• Find items with average bid 60 or more. Print the
name of the item and the average bid.
SELECT I.name, avg(B.amount)
FROM Items I, Bids B
WHERE I.iid=B.iid
GROUP BY I.iid, I.name
HAVING avg(B.amount) > 60

2. SQL
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Fall 2001 Database Systems 3
Scalar subqueries
• Find items with average bid 60 or more. Print the name
of the item and the average bid.
SELECT I.name,
FROM Items I
WHERE 60 < (SELECT avg(B.amount)
FROM bids B
WHERE B.iid = I.iid)
(SELECT avg(B2.amount)
FROM Bids B2
WHERE B2.iid = I.iid) avgAmount
Return a single
value for each tuple
Check the boolean condition
for each tuple
Fall 2001 Database Systems 4
What can’t you do with SQL
or can you?
• You are limited by the functions implemented in the
database
– Not allowed: SELECT median(salary) …
– You can add new functions to the database engine and
implement new types in a native programming language like
C,C++ (support varies)
• You can have subselects in FROM and WHERE that
return a set of values, but not in SELECT
– Allowed: SELECT * FROM (SELECT b.bid FROM bids b
WHERE b.amount>20)
– Not allowed: SELECT avg(SELECT b.amount FROM .. ) FROM
..

3. SQL
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Fall 2001 Database Systems 5
What can’t you do with SQL
or can you?
• You cannot ask transitive queries
– Given a relation “emp(A,B)” for immediate managers B or A, you cannot
find all managers of a person A.
– SQL3 includes a recursive query definition construct –not implemented
in Oracle.
WITH RECURSIVE manages(emp, mgr) AS
(SELECT E1.emp, E1.mgr FROM employee E1)
UNION
(SELECT M.emp, E2.mgr FROM manages M, employee E2
WHERE M.mgr = E.id)
SELECT M2.emp
FROM manages M2
WHERE M2.emp = M2.mgr ;
Recursive queries may produce infinite loops and it is best left to programmers
with solid logic background.
Fall 2001 Database Systems 6
Embedded SQL
• You can embed SQL statements into many programming
languages
procedural power (loops, variables, etc.)
• The main components of embedded SQL programming:
– Regular program blocks
– SQL code
– Methods to connect to the database, invoke the SQL code and
retrieve results
– Methods to pass data from program variables to the SQL code
– Methods to retrieve data from the result of queries to program
variables

4. SQL
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Fall 2001 Database Systems 7
#include <stdio.h>
exec sql include sqlca;
char user_prompt[] = "Please enter username and password: ";
char cid_prompt[] = "Please enter customer ID: ";
int main()
{
exec sql begin declare section; /* declare SQL host variables */
char cust_id[5];
char cust_name[14];
float cust_discnt; /* host var for discnt value */
char user_name[20], user_pwd[20];
exec sql end declare section;
exec sql whenever sqlerror goto report_error; /* error trap condition */
exec sql whenever not found goto notfound; /* not found condition */
exec sql connect :user_name
identified by :user_pwd; /* ORACLE format: connect */
while (prompt(cid_prompt, 1, cust_id, 4) >= 0) {
exec sql select cname,
discnt into :cust_name, :cust_discnt /* retrieve cname, discnt */
from customers where cid = :cust_id;
exec sql commit work; /* release read lock on row */
printf("CUSTOMER'S NAME IS %s AND DISCNT IS %5.1fn",
cust_name, cust_discnt); /* NOTE, (:) not used here */
continue;
Fall 2001 Database Systems 8
ProC
• Each ESQL statement starts with EXEC SQL keyword
and ends with a semicolon ;
• A pre-compiler will scan a program file and only read the
statements enclosed within EXEC SQL statements and
disregard everything else.
• SQLCA is a specific data structure for storing status
codes of all SQL operations
/* always have this for error handling*/
exec sql include sqlca ;

5. SQL
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Fall 2001 Database Systems 9
Program.pc
Program.c
Fall 2001 Database Systems 10
Variables in ESQL
• All variables that will be used in an SQL statement must be declared
using an ESQL declaration and data type
exec sql begin declare section ;
VARCHAR e_name[30], username[30], passwd[30] ;
INTEGER e_ssn, e_dept_id ;
exec sql end declare section ;
• You can use almost any SQL command in ESQL as long as proper
input to these commands are provided in the form of program
variables.
• To execute any command, you must first connect to a database in
which all your tables reside.
exec sql connect :username identified by :passwd ;

6. SQL
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Fall 2001 Database Systems 11
Executing SQL commands
• Suppose we want to find the name of an employee given his/her
SSN (input by the user of the program):
exec sql select name, dept_id into :e_name, :e_dept_id
from employee
where ssn = :e_ssn ;
Read the value of the variable “e_ssn” and execute the SQL
statement using this value, store the returned values of columns
“name” and “dept_id” in the program variables “e_name” and
“e_dept_id”.
Compare the above query with the expression below. What is the difference?
exec sql select name, dept_id
from employee where ssn = e_ssn ;
Program variables are
preceded by “:”
Fall 2001 Database Systems 12
Dealing with Strings
• There is a mismatch between the definition of a string in Oracle and
in C/C++.
– In C, the end of a string is identified by the null character ‘0’. Hence,
“Sibel” would be stored as characters ‘S’,’i’,’b’,’e’,’l’,’0’.
– In Oracle, the length of a string is stored together with the string and
there is no special end of string character.
– If you convert a data string from Oracle to C, you must pad it with ‘0’
manually!
• The data type VARCHAR e_name[30] is translated by the pre-
compiler to the following structure:
struct {
unsigned short len
unsigned char arr[30]
} e_name ;

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More on strings
Putting the pieces together:
strcpy(username.arr, “Sibel Adali”) ;
username.len = strlen(“Sibel Adali”) ;
strcpy(passwd.arr, “tweety-bird”) ;
passwd.len = strlen(“tweety-bird”) ;
exec sql connect :username identified by :passwd ;
scanf(“%d”, &e_ssn) ;
exec sql select name, dept_id into :e_name, :e_dept_id
from employee where ssn = :e_ssn ;
e_name.arr[e_name.len] = ‘0’ ; /* so can use string in C*/
printf(“%s”, e_name.arr) ;
exec sql commit work ; /* make any changes permanent */
exec sql disconnect ; /* disconnect from the database */
Fall 2001 Database Systems 14
ESQL - Cursors
• When a select statement returns a set of tuples, then the
tuples (rows) can only be retrieved one by one.
– programming language variables can contain only one
value at a time
– this is sometimes called an impedance mismatch
• Cursor operations
– declare a cursor using a regular SQL query (no “into”).
exec sql declare emps_dept cursor for
select ssn, name from employee
where dept_id = :e_dept_id ;

8. SQL
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ESQL – More Cursors
• More cursor operations
– open a cursor: means the corresponding SQL query is
executed, the results are written to a file (or a data
structure) and the cursor is pointing to the first row.
exec sql open emps_dept ;
– read the current row pointed to by the cursor using
“fetch”. At the end of fetch, the cursor is moved to point
to the next tuple.
exec sql fetch emps_dept into :e_ssn, :e_name ;
Fall 2001 Database Systems 16
How do we know when we
reach the end of a cursor?
• Each SQL statement executed in a program produces
a status code that can be retrieved from the SQL
Communication Area (SQLCA) variables.
• Check the “sqlcode” to see if the end of a cursor is
reached (its expected value depends on the system).
if (sqlca.sqlcode == -1) { … }
• Error handling statements
exec sql whenever sqlerror
{continue, stop, goto label, call function}
– exec sql whenever is a trap condition, it hold for all
exec sql statements unless a new trap condition
overrides the current one.

9. SQL
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Fall 2001 Database Systems 17
Transactions
• The most common ESQL statements are SELECT, INSERT
INTO, and UPDATE statements.
• A transaction in a program starts with the first read or write to
a database (not with connect) and ends when either one of
the following commands is executed
– exec sql commit work ; /* changes to the database
are made permanent */
– exec sql rollback work ; /* restore all tuples to their
original values */
• If a program did not complete correctly, then changes can be
undone to restore a consistent state.
Fall 2001 Database Systems 18
Dynamic SQL
• In Dynamic SQL, embedded SQL statements are created on the fly
using strings!
– these strings are fed to an exec sql statement
exec sql execute immediate :sql_string
• Since dynamic SQL statements are not known to the pre-compiler at
compile time, they must be optimized at run time!
• Create a query once using a prepare statement and run it multiple
times using the execute statement.
strcopy(sqltext.arr, “delete from employee where ssn = ?”) ;
sqltext.len=str.len(sqltext.arr) ;
exec sql prepare del_emp from :sqltext ;
exec sql execute del_emp using :cust_id ;

10. SQL
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Fall 2001 Database Systems 19
SQLDA
• When we execute a dynamic SQL statement, we do not know which
columns will be returned and how many columns will be returned.
• The SQLDA descriptor definition allows us to find the number of
columns and the value for each column.
exec sql include sqlda ;
exec sql declare sel_curs cursor for sel_emps ;
exec sql prepare sel_emps from :sqltext ;
exec sql describe sel_emps into sqlda ;
exec sql open sel_curs ;
exec sql fetch sel_curs using descriptor sqlda ;
Fall 2001 Database Systems 20
JDBC – Dynamic SQL
• Driver is a piece of software that enables
communication between a program and a
database system (DMBS specific packages).
• It implements a number of main classes:
– Connection (opening, closing, committing)
– Statement (executing queries)
– Result set (going through cursors, extracting
information)

11. SQL
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Fall 2001 Database Systems 21
import java.sql.*;
import oracle.sql.*;
import oracle.jdbc.driver.*;
class Employee
{
public static void main (String args []) throws SQLException
{//Set your user name and the password
String userName = "dummy" ;
String passWord = "dummy" ;
// Load the Oracle JDBC driver
DriverManager.registerDriver(new
oracle.jdbc.driver.OracleDriver());
Connection conn =
DriverManager.getConnection
("jdbc:oracle:thin:@vcmr-57.server.rpi.edu:1521:ora8",
userName,passWord);
Fall 2001 Database Systems 22
// Create a statement which will return a cursor that will allow
you to
// scroll the result set using both "next" and "previous"
methods
Statement stmt = conn.createStatement
(ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_READ_ONLY);
ResultSet rset = stmt.executeQuery
("SELECT name, oid FROM items ");
// Iterate through the result and print the item names
while (rset.next ()) {
//Get item name, which is the first column
System.out.println (rset.getString (1));
PreparedStatement pstmt = conn.prepareStatement
("SELECT name FROM owners WHERE oid = ?") ;
//Feed the owner id retrieved from rset into pstmt
pstmt.setInt(1, rset.getInt (2));
ResultSet dset = pstmt.executeQuery() ;
if (dset.next())
System.out.println(dset.getString (1));
} } }