Cursors, triggers, procedures

Embedded SQL Statements
 The term embedded SQL refers to SQL statements placed within an application program. Because it
houses the SQL statements, the application program is called a host program, and the language in
which it is written is called the host language. For example, Pro*C/C++ allows you to embed
certain SQL statements in a C or C++ host program.
 To manipulate and query Oracle data, you use the INSERT, UPDATE, DELETE, and SELECT
statements. INSERT adds rows of data to database tables, UPDATE modifies rows, DELETE
removes unwanted rows, and SELECT retrieves rows that meet your search condition.
 The powerful SET ROLE statement lets you dynamically manage database privileges. A role is a
named group of related system and/or object privileges granted to users or other roles. Role
definitions are stored in the Oracle data dictionary. Your applications can use the SET ROLE
statement to enable and disable roles as needed.
 Only SQL statements--not SQL*Plus statements--are valid in an application program. (SQL*Plus
has additional statements for setting environment parameters, editing, and report formatting.)

Embedded SQL
 SQL provides a powerful declarative query language. However, access to a database from a
general-purpose programming language is required because,
 SQL is not as powerful as a general-purpose programming language. There are queries
that cannot be expressed in SQL, but can be programmed in C, Fortran, Pascal, Cobol,
etc.
 Non declarative actions -- such as printing a report, interacting with a user, or sending
the result to a GUI -- cannot be done from within SQL.
 The SQL standard defines embedding of SQL as embedded SQL and the language in which
SQL queries are embedded is referred as host language.
 The result of the query is made available to the program one tuple (record) at a time.
 To identify embedded SQL requests to the preprocessor, we use EXEC SQL statement:
EXEC SQL embedded SQL statement END-EXEC
 Embedded SQL can execute any valid update, insert, or delete statements.
 Dynamic SQL component allows programs to construct and submit SQL queries ar run time

Embedded SQL Syntax
 In your application program, you can freely intermix complete SQL statements with complete C
statements and use C variables or structures in SQL statements. The only special requirement for
building SQL statements into your host program is that you begin them with the keywords
EXEC SQL and end them with a semicolon. Pro*C/C++ translates all EXEC SQL statements
into calls to the runtime library SQLLIB.
 Many embedded SQL statements differ from their interactive counterparts only through the
addition of a new clause or the use of program variables. The following example compares
interactive and embedded ROLLBACK statements:
 ROLLBACK WORK: -- interactive
 EXEC SQL ROLLBACK WORK; -- embedded
These statements have the same effect, but you would use the first in an interactive SQL
environment (such as when running SQL*Plus), and the second in a Pro*C/C++ program.

What is PL/SQL?
 PL/SQL stands for Procedural Language extension of SQL.
 PL/SQL is a combination of SQL along with the procedural features of programming
languages.
 It was developed by Oracle Corporation in the early 90’s to enhance the capabilities of SQL.
SQL (Structured Query Language) is used to modify and access data or information from
a storage area called database. This beginner online training sql tutorial website teaches
you the basics of SQL code and train you how to write & program SQL queries. I will be
sharing my database knowledge on SQL and help you learn programming SQL better. The
concepts discussed in this SQL tutorial can be applied to most of database systems. The SQL
syntax used to explain the tutorial concepts is similar to the one used in Oracle database.

My SQL Data Base
In a simple manner, SQL is a non-procedural, English-like language that processes data in
groups of records rather than one record at a time. Few functions of SQL are:
 SELECT - extracts data from a database
 UPDATE - updates data in a database
 DELETE - deletes data from a database
 INSERT INTO - inserts new data into a database
 CREATE DATABASE - creates a new database
 ALTER DATABASE - modifies a database
 CREATE TABLE - creates a new table
 ALTER TABLE - modifies a table
 DROP TABLE - deletes a table
 CREATE INDEX - creates an index (search key)
 DROP INDEX - deletes an index

Advantages of PL/SQL
These are the Advantages of PL/SQL
• Block Structures: PL SQL consists of blocks of code, which can be nested within each
other. Each block forms a unit of a task or a logical module. PL/SQL Blocks can be stored in
the database and reused.
•
Procedural Language Capability: PL SQL consists of procedural language constructs such
as conditional statements (if else statements) and loops like (FOR loops).
•
Better Performance: PL SQL engine processes multiple SQL statements simultaneously as
a single block, thereby reducing network traffic.

What are Cursors?
A cursor is a temporary work area created in the system memory when a SQL statement is executed. A
cursor contains information on a select statement and the rows of data accessed by it.
This temporary work area is used to store the data retrieved from the database, and manipulate this
data. A cursor can hold more than one row, but can process only one row at a time. The set of rows the
cursor holds is called the active set.
There are two types of cursors in PL/SQL:
Implicit cursors
These are created by default when DML statements like, INSERT, UPDATE, and DELETE statements
are executed. They are also created when a SELECT statement that returns just one row is executed.
Explicit cursors
They must be created when you are executing a SELECT statement that returns more than one row.
Even though the cursor stores multiple records, only one record can be processed at a time, which is
called as current row. When you fetch a row the current row position moves to next row.
Both implicit and explicit cursors have the same functionality, but they differ in the way they are
accessed.

Implicit Cursors: Application
 When you execute DML statements like DELETE, INSERT, UPDATE and SELECT
statements, implicit statements are created to process these statements.
 Oracle provides few attributes called as implicit cursor attributes to check the status of
DML operations. The cursor attributes available are %FOUND, %NOTFOUND,
%ROWCOUNT, and %ISOPEN.
 For example,
When you execute INSERT, UPDATE, or DELETE statements the cursor attributes tell us
whether any rows are affected and how many have been affected.
When a SELECT... INTO statement is executed in a PL/SQL Block, implicit cursor attributes
can be used to find out whether any row has been returned by the SELECT statement.
PL/SQL returns an error when no data is selected.

Status of the cursor for each of these attributes are defined in the below table.
Attributes Return Value Example
%FOUND The return value is TRUE, if the DML statements
like INSERT, DELETE and UPDATE affect at least one
row and if SELECT ….INTO statement return at least
one row.
SQL%FOUND
The return value is FALSE, if DML statements like
INSERT, DELETE and UPDATE do not affect row and
if SELECT….INTO statement do not return a row.
%NOTFOUND The return value is FALSE, if DML statements like
INSERT, DELETE and UPDATE at least one row and if
SELECT ….INTO statement return at least one row.
SQL%NOTFOUND
The return value is TRUE, if a DML statement like
INSERT, DELETE and UPDATE do not affect even one
row and if SELECT ….INTO statement does not
return a row.
%ROWCOUNT Return the number of rows affected by the DML
operations INSERT, DELETE, UPDATE, SELECT
SQL%ROWCOUNT

What is a Trigger?
A trigger is a pl/sql block structure which is fired when a DML statements like Insert, Delete,
Update is executed on a database table. A trigger is triggered automatically when an
associated DML statement is executed.
Syntax of Triggers
Syntax for Creating a Trigger
CREATE [OR REPLACE ] TRIGGER trigger_name
{BEFORE | AFTER | INSTEAD OF }
{INSERT [OR] | UPDATE [OR] | DELETE}
[OF col_name]
ON table_name
[REFERENCING OLD AS o NEW AS n]
[FOR EACH ROW]
WHEN (condition)
BEGIN
--- sql statements
END;

 CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or
overwrites an existing trigger with the same name.
 {BEFORE | AFTER | INSTEAD OF } - This clause indicates at what time should the trigger get fired. i.e for
example: before or after updating a table. INSTEAD OF is used to create a trigger on a view. before and
after cannot be used to create a trigger on a view.
 {INSERT [OR] | UPDATE [OR] | DELETE} - This clause determines the triggering event. More than one
triggering events can be used together separated by OR keyword. The trigger gets fired at all the
specified triggering event.
 [OF col_name] - This clause is used with update triggers. This clause is used when you want to trigger an
event only when a specific column is updated.
 CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or
overwrites an existing trigger with the same name.
 [ON table_name] - This clause identifies the name of the table or view to which the trigger is associated.
 [REFERENCING OLD AS o NEW AS n] - This clause is used to reference the old and new values of the data
being changed. By default, you reference the values as :old.column_name or :new.column_name. The
reference names can also be changed from old (or new) to any other user-defined name. You cannot
reference old values when inserting a record, or new values when deleting a record, because they do not
exist.
 [FOR EACH ROW] - This clause is used to determine whether a trigger must fire when each row gets
affected ( i.e. a Row Level Trigger) or just once when the entire sql statement is executed(i.e.statement
level Trigger)
 WHEN (condition) - This clause is valid only for row level triggers. The trigger is fired only for rows that
satisfy the condition specified.

Types of PL/SQL Triggers
There are two types of triggers based on the which level it is triggered.
1) Row level trigger - An event is triggered for each row upated, inserted or deleted.
2) Statement level trigger - An event is triggered for each sql statement executed.
 PL/SQL Trigger Execution Hierarchy
 The following hierarchy is followed when a trigger is fired
1) BEFORE statement trigger fires first.
2) Next BEFORE row level trigger fires, once for each row affected.
3) Then AFTER row level trigger fires once for each affected row. This events will
alternates between BEFORE and AFTER row level triggers.
4) Finally the AFTER statement level trigger fires.

What is a Stored Procedure?
A stored procedure or in simple a proc is a named PL/SQL block which performs one or more specific
task. This is similar to a procedure in other programming languages.
 A procedure has a header and a body. The header consists of the name of the procedure and the
parameters or variables passed to the procedure. The body consists or declaration
section, execution section and exception section similar to a general PL/SQL Block.
 A procedure is similar to an anonymous PL/SQL Block but it is named for repeated usage.
We can pass parameters to procedures in three ways.
1) IN-parameters
2) OUT-parameters
3) IN OUT-parameter

General Syntax to create a procedure is:
CREATE [OR REPLACE] PROCEDURE proc_name [list of parameters]
IS
Declaration section
BEGIN
Execution section
EXCEPTION
Exception section
END;
How to execute a Stored Procedure?
There are two ways to execute a procedure.
1) From the SQL prompt.
EXECUTE [or EXEC] procedure_name;
2) Within another procedure – simply use the procedure name.
procedure_name;

Cursors, triggers, procedures

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