Embedded SQL
Unit 5.1
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
1
Interactive SQL
So far in the module we have considered only the SQL queries
which you can type in at the SQL prompt. We refer to this as
‘interactive’ SQL.
 SQL is a ‘non-procedural’ language
 SQL specifies WHAT is required, not HOW this requirement is
to be met.
 INTERACTIVE SQL is good for:
– defining database structure
– generating low-volume, ad hoc queries
– prototyping
 INTERACTIVE SQL is not good for the more sophisticated
applications for which a programming language with links to
SQL might be better.
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
2
Embedded SQL
SQL can be embedded within procedural programming
languages. These language (sometimes referred to as 3GLs)
include C/C++, Cobol, Fortran, and Perl. Thus the embedded
SQL provides the 3GL with a way to manipulate a database,
supporting:
 highly customized applications
 background applications running without user intervention
 database manipulation which exceeds the abilities of simple
SQL
 applications linking to Oracle packages, e.g. forms and
reports
 applications which need customized window interfaces
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
3
SQL Precompiler
Precompilers are used to translate
SQL statements embedded in a host
language into DBMS library calls
which can be implemented in the
host language. This method of
embedding was common where the
links between database and program
were to be blurred. In recent times,
the database-program link is shown
in the program much more explicitly,
and the need for precompilers has
been greatly reduced.
Dr Gordon Russell, Copyright
@ Napier University
Editor
host program+ embedded SQL
Precompiler
host program+ translated SQL
Compiler
Linker
Unit 5.1 - Embedded SQL - V3.0
object (binary) program
DBMS and other libraries
executable program
4
Sharing Variables
In general the variables of the program are isolated from the
variables (the rows and columns) of the database system. This
gives the programmer a much better understanding of what is
actually being changed when a variable is set.
Some variables are still globally shared between databases and
programming languages. These are mostly to do with error
tracking. For instance, executing an SQL statement which results
in an error may return an error code, but a global variable may
also be set with a much more detailed error report.
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
5
Connecting to the
DBMS
In this lecture, Perl examples are used. However, no
understanding of Perl should be necessary to understand the
examples…
Connecting to the database is little more than:
$dbh = DBI->connect(“dbname”,”username”,”password”)
This gives an “handle” on the database connection which can be
used to instigate further commands.
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
6
Executing SQL
SQL to be executed goes through a two phase process of
interpretation (prepare) and then execution (execute).
$cmd = $dbh->prepare(“
SELECT custname FROM customers
WHERE cno = ?”)
$res = $cmd->execute(10)
Thus $res is a link to the results of executing this SQL where the
? Is replaced with the number “10”.
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
7
Cursors - SELECT
many rows

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A cursor provides a pointer to a single row in the result of a
selection query (which may return may rows)
One row at a time is accessed through the cursor, which is
moved to the next row before each data transfer
The columns of that one row are ‘fetched’ into the program
variables which can then be manipulated in the normal way
by the host program.
A cursor can also be used to update values in tables if it is
linked to an INSERT SQL command rather than a SELECT
query.
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
8
Simple SELECT Cursor
$cmd = $dbh->prepare(“SELECT custname FROM customers
WHERE cno = ?”)
$res = $cmd->execute(10)
While ( ($name) = $res->fetchrow_array()) {
print “The customer name is $name”
}
This prints out all the names of customers where the cno is 10.
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
9
Simple INSERT cursor
$cmd = $dbh->prepare(“INSERT into EMPCOURSE values (?,?)”);
@records = ([10,20],[10,30]);
For $r (@records) {
$cmd->bind_param(1,$r->[0],SQL_INTEGER);
$cmd->bind_param(2,$r->[1],SQL_INTEGER);
$cmd->execute();
$dbh->commit();
}
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
10
Summary




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Cursors provide a means of integrating traditional 3GL
procedural languages and databases by enabling row-at-atime access.
Languages such as Visual Basic and Visual C++ also have
build-in statements that enable this type of processing with a
dedicated database, like that provided by MS-Access.
Java has a well-defined interface to enable easy access to
databases through a Database Connectivity library - JDBC.
Perl makes use of the DBI standard, which is fast becoming
the main contender to ODBC.
Unfortunately, there are many ‘standards’.
Dr Gordon Russell, Copyright
@ Napier University
Unit 5.1 - Embedded SQL - V3.0
11