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Embedded SQL
John Ortiz
Why Isn’t Interactive SQL Enough?
 How to do this using interactive SQL?
Print a well-formatted transcript of a
student with either a name or an id
 What does it take for someone to use SQL?
To know schemas, syntax, semantics,
mathematics, logic, …
 Solution?
Write application programs to help naïve users
to manipulate the data.
 How to access database from within a
program?
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Idea of Embedded SQL
 Combine the power of both SQL & a general
purpose programming language.
Use (embedded) SQL to perform data
retrieval and updates.
Use the general purpose programming
language (host PL) to perform more
complex data processing and to provide a
friendly user interface.
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Oracle API
 Support embedded SQL through five host PLs
(pro*languages)
C/C++, Cobol, PL/I, Ada, Pascal
 Oracle8i supports Java/JDBC and SQLJ
 SQL stmts are placed in host PL programs
 Data flow from database to program variables
and vice versa
 Two step compilation:
Precompilation: prog.pc  prog.cc
Compilation: prog.cc  prog.o
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A Sample Pro*C/C++ Program
 The program is sample1.pc
 Common tasks:
Declare variables interfacing SQL & host PL
Prepare for any SQL error
Include sqlca (communication area)
Use whenever sqlerror, …
Provide for error processing
Connect to database
Issue SQL statements
Disconnect the database
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PL & SQL Variables
 Every variable used to get data from & pass
data to database must be declared in a declare
section
exec sql begin declare section;
varchar user_name[20], passwd[10];
…
exec sql end declare section;
 Must use SQL data types
 Can appear in both SQL (preceded by a colon
:user_name) & PL statements (no colon)
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PL & SQL Variables (cont.)
 Oracle Pro*C pre-processor will convert
varchar user_name[20];
to:
struct {
unsigned short len;
unsigned char arr[20];
} user_name;
 SQL string is not ‘\0’-ended, but C string is
 Variables must have comparable types to get
data from table columns
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Handle SQL Errors
 SQLCA (p.121): SQL Communication area is a
predefined data structure used to pass control
information from Oracle to application program
Error code:
sqlca.sqlcode=0 successful;
> 0 value not found; < 0 error
 Can also use predefined error types in
whenever: sqlerror, not found, sqlwarning, …
 Common error handling actions: goto Label,
continue, stop, do func, do break, do return, …
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Embedded SQL Statements
 Every SQL statement is preceded by exec sql
 Can use all SQL statements plus special ones.
Connect, disconnect
Whenever
Select … into … from …
Rollback
Commit
Statements declare and use cursors
Statements define and execute dynamic
queries
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Transaction Control Statements
 A transaction is modeled by a sequence of
database update operations
 Operations of a transaction should either all be
successfully executed or none is executed
 When a failure occurs, all updates done so far
need to be undone
 Explicit control:
Start: set transaction [read only, read write]
Undo: rollback
End: commit
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Sample Program Using A Cursor
 How does a program handle query result
containing more than one tuple?
 Use a cursor. See sample2.pc
 A cursor is a “window” through which one tuple
can be accessed.
 A cursor must be declared with a query
 Open cursor executes the query
 Fetch cursor moves to the next tuple
 A cursor can be closed and re-opened
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Dynamic SQL
 Create SQL statements at run time and then
execute the newly created statements.
 General framework:
Declare a host string variable.
Place an SQL statement in the variable at
run-time.
Let the DBMS parse & execute the SQL
statement in the host variable.
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Why Dynamic SQL
 Consider relation:
Projects(Pno, Name, Budget, Duration)
 How to write a program to delete projects
that satisfy some yet unknown conditions?
delete Projects where <unknown>
 Some possible types of conditions:
(1) Budget > 2000000
(2) Budget > 2000000 and Duration > 24
(3) Name = 'Manned Spacecraft to Mars‘
 Why isn’t static SQL a viable solution?
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Methods of Dynamic SQL
 Method 1: Non-query statement w/o host var.
delete from emp where deptno = 20
 Method 2: Non-query w/ known # of input var.
delete from emp where empno = :eid
 Method 3: Query w/ known select-list & input
host variables.
select ename from emp where deptno = :dno
 Method 4: Query w/ unknown select-list & var.
select <unknown> from emp
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Execute-Immediate Statement
 A sample program
 execute immediate causes the dynamic SQL
statement to be compiled and executed
immediately.
 General execute immediate statement:
exec sql execute immediate :host_variable;
 Implement Method 1.
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Prepare-Execute Statement
 A sample program
 prepare stmt_name from :host_variable
compiles the SQL query in host_variable and
then saves the prepared result in stmt_name.
stmt_name is an SQL identifier and needs
not be explicitly declared.
 Implement Method 2: The host_variable can
contain any SQL statement other than a query
and can have a known number of placeholders.
 Prepare once, run many times (with different
values in input host variables).
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Pro*C/C++ and PL/SQL
 On Oracle, Pro*C/C++ programs may contain
any SQL statement and PL/SQL blocks.
 See this sample program
 Precompiled and stored PL/SQL procedures
and functions can be used directly in
embedded SQL statement
exec sql execute p(…)
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Oracle JDBC
 JDBC (Java Database Connectivity): API
that allows Java applications and applets to
connect to Oracle databases, send SQL
statements, and receive data from
databases.
 Need to set up CLASSPATH environment
variable.
 See QueryUnivDB.java
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JDBC: Program Tasks
 Import Java SQL API library
 Load JDBC driver: Class.forName(…)
 Create a DB connection
DriveManager.getConnection(<connect string>)
 Create a query statement object
 Create a result object by executing the query
 Process the result
 Close the query
 Close the connection
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Oracle SQLJ
 Java with embedded SQL (prec. w/ #sql)
 Part of the new SQL 1999 Standard.
 Much easier to use:
Connect to database
Create iterators for query results
 Oracle8i only supports JDK1.1
 Need two compilation steps:
sqlj file[.sqlj]
javac file.java
 See QueryUnivDB.sqlj
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Embedded SQL Summary
 Many additional statements handling
communication between database and PL
system
 Special handling of multiple tuple result
(cursor)
 Must handle errors and exceptions generated
by DBMS
 Must pay special attention to program
structure (goto, whenever not found,… )
 JDBC & SQLJ have much better DB-PL
interface
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Look Ahead
 Next topic: Storage System
 Read textbook:
Chapter 5
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