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
The Syntax for creating a trigger is:
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.
For Example: The price of a product changes constantly. It is important to maintain the history of the prices of the products.
We can create a trigger to update the 'product_price_history' table when the price of the product is updated in the 'product' table.
1) Create the 'product' table and 'product_price_history' table
CREATE TABLE product_price_history
(product_id number(5),
product_name varchar2(32),
supplier_name varchar2(32),
unit_price number(7,2) );
CREATE TABLE product
(product_id number(5),
product_name varchar2(32),
supplier_name varchar2(32),
unit_price number(7,2) );
2) Create the price_history_trigger and execute it.
CREATE or REPLACE TRIGGER price_history_trigger
BEFORE UPDATE OF unit_price
ON product
FOR EACH ROW
BEGIN
INSERT INTO product_price_history
VALUES
(:old.product_id,
:old.product_name,
:old.supplier_name,
:old.unit_price);
END;
/
3) Lets update the price of a product.
UPDATE PRODUCT SET unit_price = 800 WHERE product_id = 100
Once the above update query is executed, the trigger fires and updates the 'product_price_history' table.
4)If you ROLLBACK the transaction before committing to the database, the data inserted to the table is also rolled back.
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.
For Example: Let's create a table 'product_check' which we can use to store messages when triggers are fired.
CREATE TABLE product
(Message varchar2(50),
Current_Date number(32)
);
Let's create a BEFORE and AFTER statement and row level triggers for the product table.
1) BEFORE UPDATE, Statement Level: This trigger will insert a record into the table 'product_check' before a sql update statement is executed, at the statement level.
CREATE or REPLACE TRIGGER Before_Update_Stat_product
BEFORE
UPDATE ON product
Begin
INSERT INTO product_check
Values('Before update, statement level',sysdate);
END;
/
2) BEFORE UPDATE, Row Level: This trigger will insert a record into the table 'product_check' before each row is updated.
CREATE or REPLACE TRIGGER Before_Upddate_Row_product
BEFORE
UPDATE ON product
FOR EACH ROW
BEGIN
INSERT INTO product_check
Values('Before update row level',sysdate);
END;
/
3) AFTER UPDATE, Statement Level: This trigger will insert a record into the table 'product_check' after a sql update statement is executed, at the statement level.
CREATE or REPLACE TRIGGER After_Update_Stat_product
AFTER
UPDATE ON product
BEGIN
INSERT INTO product_check
Values('After update, statement level', sysdate);
End;
/
4) AFTER UPDATE, Row Level: This trigger will insert a record into the table 'product_check' after each row is updated.
CREATE or REPLACE TRIGGER After_Update_Row_product
AFTER
insert On product
FOR EACH ROW
BEGIN
INSERT INTO product_check
Values('After update, Row level',sysdate);
END;
/
Now lets execute a update statement on table product.
UPDATE PRODUCT SET unit_price = 800
WHERE product_id in (100,101);
Lets check the data in 'product_check' table to see the order in which the trigger is fired.
SELECT * FROM product_check;
Output:
Mesage Current_Date
------------------------------------------------------------
Before update, statement level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
Before update, row level 26-Nov-2008
After update, Row level 26-Nov-2008
After update, statement level 26-Nov-2008
The above result shows 'before update' and 'after update' row level events have occured twice, since two records were updated. But 'before update' and 'after update' statement level events are fired only once per sql statement.
The above rules apply similarly for INSERT and DELETE statements.
How To know Information about Triggers.
We can use the data dictionary view 'USER_TRIGGERS' to obtain information about any trigger.
The below statement shows the structure of the view 'USER_TRIGGERS'
DESC USER_TRIGGERS;
NAME Type
--------------------------------------------------------
TRIGGER_NAME VARCHAR2(30)
TRIGGER_TYPE VARCHAR2(16)
TRIGGER_EVENT VARCHAR2(75)
TABLE_OWNER VARCHAR2(30)
BASE_OBJECT_TYPE VARCHAR2(16)
TABLE_NAME VARCHAR2(30)
COLUMN_NAME VARCHAR2(4000)
REFERENCING_NAMES VARCHAR2(128)
WHEN_CLAUSE VARCHAR2(4000)
STATUS VARCHAR2(8)
DESCRIPTION VARCHAR2(4000)
ACTION_TYPE VARCHAR2(11)
TRIGGER_BODY LONG
This view stores information about header and body of the trigger.
SELECT * FROM user_triggers WHERE trigger_name = 'Before_Update_Stat_product';
The above sql query provides the header and body of the trigger 'Before_Update_Stat_product'.
You can drop a trigger using the following command.
DROP TRIGGER trigger_name;
CYCLIC CASCADING in a TRIGGER
This is an undesirable situation where more than one trigger enter into an infinite loop. while creating a trigger we should ensure the such a situtation does not exist.
The below example shows how Trigger's can enter into cyclic cascading.
Let's consider we have two tables 'abc' and 'xyz'. Two triggers are created.
1) The INSERT Trigger, triggerA on table 'abc' issues an UPDATE on table 'xyz'.
2) The UPDATE Trigger, triggerB on table 'xyz' issues an INSERT on table 'abc'.
In such a situation, when there is a row inserted in table 'abc', triggerA fires and will update table 'xyz'.
When the table 'xyz' is updated, triggerB fires and will insert a row in table 'abc'.
This cyclic situation continues and will enter into a infinite loop, which will crash the database.
================
Stored Procedures
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-parameters
A procedure may or may not return any value.
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;
IS - marks the beginning of the body of the procedure and is similar to DECLARE in anonymous PL/SQL Blocks. The code between IS and BEGIN forms the Declaration section.
The syntax within the brackets [ ] indicate they are optional. By using CREATE OR REPLACE together the procedure is created if no other procedure with the same name exists or the existing procedure is replaced with the current code.
The below example creates a procedure ‘employer_details’ which gives the details of the employee.
1> CREATE OR REPLACE PROCEDURE employer_details
2> IS
3> CURSOR emp_cur IS
4> SELECT first_name, last_name, salary FROM emp_tbl;
5> emp_rec emp_cur%rowtype;
6> BEGIN
7> FOR emp_rec in sales_cur
8> LOOP
9> dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
10> || ' ' ||emp_cur.salary);
11> END LOOP;
12>END;
13> /
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;
NOTE: In the examples given above, we are using backward slash ‘/’ at the end of the program. This indicates the oracle engine that the PL/SQL program has ended and it can begin processing the statements.
========
PL/SQL Functions
What is a Function in PL/SQL?
A function is a named PL/SQL Block which is similar to a procedure. The major difference between a procedure and a function is, a function must always return a value, but a procedure may or may not return a value.
The General Syntax to create a function is:
CREATE [OR REPLACE] FUNCTION function_name [parameters]
RETURN return_datatype;
IS
Declaration_section
BEGIN
Execution_section
Return return_variable;
EXCEPTION
exception section
Return return_variable;
END;
1) Return Type: The header section defines the return type of the function. The return datatype can be any of the oracle datatype like varchar, number etc.
2) The execution and exception section both should return a value which is of the datatype defined in the header section.
For example, let’s create a frunction called ''employer_details_func' similar to the one created in stored proc
1> CREATE OR REPLACE FUNCTION employer_details_func
2> RETURN VARCHAR(20);
3> IS
5> emp_name VARCHAR(20);
6> BEGIN
7> SELECT first_name INTO emp_name
8> FROM emp_tbl WHERE empID = '100';
9> RETURN emp_name;
10> END;
11> /
In the example we are retrieving the ‘first_name’ of employee with empID 100 to variable ‘emp_name’.
The return type of the function is VARCHAR which is declared in line no 2.
The function returns the 'emp_name' which is of type VARCHAR as the return value in line no 9.
How to execute a PL/SQL Function?
A function can be executed in the following ways.
1) Since a function returns a value we can assign it to a variable.
employee_name := employer_details_func;
If ‘employee_name’ is of datatype varchar we can store the name of the employee by assigning the return type of the function to it.
2) As a part of a SELECT statement
SELECT employer_details_func FROM dual;
3) In a PL/SQL Statements like,
dbms_output.put_line(employer_details_func);
This line displays the value returned by the function.
=========
Parameters in Procedure and Functions
How to pass parameters to Procedures and Functions in PL/SQL ?
In PL/SQL, we can pass parameters to procedures and functions in three ways.
1) IN type parameter: These types of parameters are used to send values to stored procedures.
2) OUT type parameter: These types of parameters are used to get values from stored procedures. This is similar to a return type in functions.
3) IN OUT parameter: These types of parameters are used to send values and get values from stored procedures.
NOTE: If a parameter is not explicitly defined a parameter type, then by default it is an IN type parameter.
1) IN parameter:
This is similar to passing parameters in programming languages. We can pass values to the stored procedure through these parameters or variables. This type of parameter is a read only parameter. We can assign the value of IN type parameter to a variable or use it in a query, but we cannot change its value inside the procedure.
The General syntax to pass a IN parameter is
CREATE [OR REPLACE] PROCEDURE procedure_name (
param_name1 IN datatype, param_name12 IN datatype ... )
param_name1, param_name2... are unique parameter names.
datatype - defines the datatype of the variable.
IN - is optional, by default it is a IN type parameter.
2) OUT Parameter:
The OUT parameters are used to send the OUTPUT from a procedure or a function. This is a write-only parameter i.e, we cannot pass values to OUT paramters while executing the stored procedure, but we can assign values to OUT parameter inside the stored procedure and the calling program can recieve this output value.
The General syntax to create an OUT parameter is
CREATE [OR REPLACE] PROCEDURE proc2 (param_name OUT datatype)
The parameter should be explicity declared as OUT parameter.
3) IN OUT Parameter:
The IN OUT parameter allows us to pass values into a procedure and get output values from the procedure. This parameter is used if the value of the IN parameter can be changed in the calling program.
By using IN OUT parameter we can pass values into a parameter and return a value to the calling program using the same parameter. But this is possible only if the value passed to the procedure and output value have a same datatype. This parameter is used if the value of the parameter will be changed in the procedure.
The General syntax to create an IN OUT parameter is
CREATE [OR REPLACE] PROCEDURE proc3 (param_name IN OUT datatype)
The below examples show how to create stored procedures using the above three types of parameters.
Example1:
Using IN and OUT parameter:
Let’s create a procedure which gets the name of the employee when the employee id is passed.
1> CREATE OR REPLACE PROCEDURE emp_name (id IN NUMBER, emp_name OUT NUMBER)
2> IS
3> BEGIN
4> SELECT first_name INTO emp_name
5> FROM emp_tbl WHERE empID = id;
6> END;
7> /
We can call the procedure ‘emp_name’ in this way from a PL/SQL Block.
1> DECLARE
2> empName varchar(20);
3> CURSOR id_cur SELECT id FROM emp_ids;
4> BEGIN
5> FOR emp_rec in id_cur
6> LOOP
7> emp_name(emp_rec.id, empName);
8> dbms_output.putline('The employee ' || empName || ' has id ' || emp-rec.id);
9> END LOOP;
10> END;
11> /
In the above PL/SQL Block
In line no 3; we are creating a cursor ‘id_cur’ which contains the employee id.
In line no 7; we are calling the procedure ‘emp_name’, we are passing the ‘id’ as IN parameter and ‘empName’ as OUT parameter.
In line no 8; we are displaying the id and the employee name which we got from the procedure ‘emp_name’.
Example 2:
Using IN OUT parameter in procedures:
1> CREATE OR REPLACE PROCEDURE emp_salary_increase
2> (emp_id IN emptbl.empID%type, salary_inc IN OUT emptbl.salary%type)
3> IS
4> tmp_sal number;
5> BEGIN
6> SELECT salary
7> INTO tmp_sal
8> FROM emp_tbl
9> WHERE empID = emp_id;
10> IF tmp_sal between 10000 and 20000 THEN
11> salary_inout := tmp_sal * 1.2;
12> ELSIF tmp_sal between 20000 and 30000 THEN
13> salary_inout := tmp_sal * 1.3;
14> ELSIF tmp_sal > 30000 THEN
15> salary_inout := tmp_sal * 1.4;
16> END IF;
17> END;
18> /
The below PL/SQL block shows how to execute the above 'emp_salary_increase' procedure.
1> DECLARE
2> CURSOR updated_sal is
3> SELECT empID,salary
4> FROM emp_tbl;
5> pre_sal number;
6> BEGIN
7> FOR emp_rec IN updated_sal LOOP
8> pre_sal := emp_rec.salary;
9> emp_salary_increase(emp_rec.empID, emp_rec.salary);
10> dbms_output.put_line('The salary of ' || emp_rec.empID ||
11> ' increased from '|| pre_sal || ' to '||emp_rec.salary);
12> END LOOP;
13> END;
14> /
==========
Exception Handling
In this section we will discuss about the following,
1) What is Exception Handling.
2) Structure of Exception Handling.
3) Types of Exception Handling.
1) What is Exception Handling?
PL/SQL provides a feature to handle the Exceptions which occur in a PL/SQL Block known as exception Handling. Using Exception Handling we can test the code and avoid it from exiting abruptly. When an exception occurs a messages which explains its cause is recieved.
PL/SQL Exception message consists of three parts.
1) Type of Exception
2) An Error Code
3) A message
By Handling the exceptions we can ensure a PL/SQL block does not exit abruptly.
2) Structure of Exception Handling.
The General Syntax for coding the exception section
DECLARE
Declaration section
BEGIN
Exception section
EXCEPTION
WHEN ex_name1 THEN
-Error handling statements
WHEN ex_name2 THEN
-Error handling statements
WHEN Others THEN
-Error handling statements
END;
General PL/SQL statments can be used in the Exception Block.
When an exception is raised, Oracle searches for an appropriate exception handler in the exception section. For example in the above example, if the error raised is 'ex_name1 ', then the error is handled according to the statements under it. Since, it is not possible to determine all the possible runtime errors during testing fo the code, the 'WHEN Others' exception is used to manage the exceptions that are not explicitly handled. Only one exception can be raised in a Block and the control does not return to the Execution Section after the error is handled.
If there are nested PL/SQL blocks like this.
DELCARE
Declaration section
BEGIN
DECLARE
Declaration section
BEGIN
Execution section
EXCEPTION
Exception section
END;
EXCEPTION
Exception section
END;
In the above case, if the exception is raised in the inner block it should be handled in the exception block of the inner PL/SQL block else the control moves to the Exception block of the next upper PL/SQL Block. If none of the blocks handle the exception the program ends abruptly with an error.
3) Types of Exception.
There are 3 types of Exceptions.
a) Named System Exceptions
b) Unnamed System Exceptions
c) User-defined Exceptions
a) Named System Exceptions
System exceptions are automatically raised by Oracle, when a program violates a RDBMS rule. There are some system exceptions which are raised frequently, so they are pre-defined and given a name in Oracle which are known as Named System Exceptions.
For example: NO_DATA_FOUND and ZERO_DIVIDE are called Named System exceptions.
Named system exceptions are:
1) Not Declared explicitly,
2) Raised implicitly when a predefined Oracle error occurs,
3) caught by referencing the standard name within an exception-handling routine.
Exception Name Reason Error Number
CURSOR_ALREADY_OPEN
When you open a cursor that is already open.
ORA-06511
INVALID_CURSOR
When you perform an invalid operation on a cursor like closing a cursor, fetch data from a cursor that is not opened.
ORA-01001
NO_DATA_FOUND
When a SELECT...INTO clause does not return any row from a table.
ORA-01403
TOO_MANY_ROWS
When you SELECT or fetch more than one row into a record or variable.
ORA-01422
ZERO_DIVIDE
When you attempt to divide a number by zero.
ORA-01476
For Example: Suppose a NO_DATA_FOUND exception is raised in a proc, we can write a code to handle the exception as given below.
BEGIN
Execution section
EXCEPTION
WHEN NO_DATA_FOUND THEN
dbms_output.put_line ('A SELECT...INTO did not return any row.');
END;
b) Unnamed System Exceptions
Those system exception for which oracle does not provide a name is known as unamed system exception. These exception do not occur frequently. These Exceptions have a code and an associated message.
There are two ways to handle unnamed sysyem exceptions:
1. By using the WHEN OTHERS exception handler, or
2. By associating the exception code to a name and using it as a named exception.
We can assign a name to unnamed system exceptions using a Pragma called EXCEPTION_INIT.
EXCEPTION_INIT will associate a predefined Oracle error number to a programmer_defined exception name.
Steps to be followed to use unnamed system exceptions are
• They are raised implicitly.
• If they are not handled in WHEN Others they must be handled explicity.
• To handle the exception explicity, they must be declared using Pragma EXCEPTION_INIT as given above and handled referecing the user-defined exception name in the exception section.
The general syntax to declare unnamed system exception using EXCEPTION_INIT is:
DECLARE
exception_name EXCEPTION;
PRAGMA
EXCEPTION_INIT (exception_name, Err_code);
BEGIN
Execution section
EXCEPTION
WHEN exception_name THEN
handle the exception
END;
For Example: Lets consider the product table and order_items table from sql joins.
Here product_id is a primary key in product table and a foreign key in order_items table.
If we try to delete a product_id from the product table when it has child records in order_id table an exception will be thrown with oracle code number -2292.
We can provide a name to this exception and handle it in the exception section as given below.
DECLARE
Child_rec_exception EXCEPTION;
PRAGMA
EXCEPTION_INIT (Child_rec_exception, -2292);
BEGIN
Delete FROM product where product_id= 104;
EXCEPTION
WHEN Child_rec_exception
THEN Dbms_output.put_line('Child records are present for this product_id.');
END;
/
c) User-defined Exceptions
Apart from sytem exceptions we can explicity define exceptions based on business rules. These are known as user-defined exceptions.
Steps to be followed to use user-defined exceptions:
• They should be explicitly declared in the declaration section.
• They should be explicitly raised in the Execution Section.
• They should be handled by referencing the user-defined exception name in the exception section.
For Example: Lets consider the product table and order_items table from sql joins to explain user-defined exception.
Lets create a business rule that if the total no of units of any particular product sold is more than 20, then it is a huge quantity and a special discount should be provided.
DECLARE
huge_quantity EXCEPTION;
CURSOR product_quantity is
SELECT p.product_name as name, sum(o.total_units) as units
FROM order_tems o, product p
WHERE o.product_id = p.product_id;
quantity order_tems.total_units%type;
up_limit CONSTANT order_tems.total_units%type := 20;
message VARCHAR2(50);
BEGIN
FOR product_rec in product_quantity LOOP
quantity := product_rec.units;
IF quantity > up_limit THEN
message := 'The number of units of product ' || product_rec.name ||
' is more than 20. Special discounts should be provided.
Rest of the records are skipped. '
RAISE huge_quantity;
ELSIF quantity < up_limit THEN
v_message:= 'The number of unit is below the discount limit.';
END IF;
dbms_output.put_line (message);
END LOOP;
EXCEPTION
WHEN huge_quantity THEN
dbms_output.put_line (message);
END;
/
RAISE_APPLICATION_ERROR ( )
RAISE_APPLICATION_ERROR is a built-in procedure in oracle which is used to display the user-defined error messages along with the error number whose range is in between -20000 and -20999.
Whenever a message is displayed using RAISE_APPLICATION_ERROR, all previous transactions which are not committed within the PL/SQL Block are rolled back automatically (i.e. change due to INSERT, UPDATE, or DELETE statements).
RAISE_APPLICATION_ERROR raises an exception but does not handle it.
RAISE_APPLICATION_ERROR is used for the following reasons,
a) to create a unique id for an user-defined exception.
b) to make the user-defined exception look like an Oracle error.
The General Syntax to use this procedure is:
RAISE_APPLICATION_ERROR (error_number, error_message);
• The Error number must be between -20000 and -20999
• The Error_message is the message you want to display when the error occurs.
Steps to be folowed to use RAISE_APPLICATION_ERROR procedure:
1. Declare a user-defined exception in the declaration section.
2. Raise the user-defined exception based on a specific business rule in the execution section.
3. Finally, catch the exception and link the exception to a user-defined error number in RAISE_APPLICATION_ERROR.
Using the above example we can display a error message using RAISE_APPLICATION_ERROR.
DECLARE
huge_quantity EXCEPTION;
CURSOR product_quantity is
SELECT p.product_name as name, sum(o.total_units) as units
FROM order_tems o, product p
WHERE o.product_id = p.product_id;
quantity order_tems.total_units%type;
up_limit CONSTANT order_tems.total_units%type := 20;
message VARCHAR2(50);
BEGIN
FOR product_rec in product_quantity LOOP
quantity := product_rec.units;
IF quantity > up_limit THEN
RAISE huge_quantity;
ELSIF quantity < up_limit THEN
v_message:= 'The number of unit is below the discount limit.';
END IF;
Dbms_output.put_line (message);
END LOOP;
EXCEPTION
WHEN huge_quantity THEN
raise_application_error(-2100, 'The number of unit is above the discount limit.');
END;
/
==========
PL/SQL Records
What are records?
Records are another type of datatypes which oracle allows to be defined as a placeholder. Records are composite datatypes, which means it is a combination of different scalar datatypes like char, varchar, number etc. Each scalar data types in the record holds a value. A record can be visualized as a row of data. It can contain all the contents of a row.
Declaring a record:
To declare a record, you must first define a composite datatype; then declare a record for that type.
The General Syntax to define a composite datatype is:
TYPE record_type_name IS RECORD
(first_col_name column_datatype,
second_col_name column_datatype, ...);
record_type_name – it is the name of the composite type you want to define.
first_col_name, second_col_name, etc.,- it is the names the fields/columns within the record.
column_datatype defines the scalar datatype of the fields.
There are different ways you can declare the datatype of the fields.
1) You can declare the field in the same way as you declare the fieds while creating the table.
2) If a field is based on a column from database table, you can define the field_type as follows:
col_name table_name.column_name%type;
By declaring the field datatype in the above method, the datatype of the column is dynamically applied to the field. This method is useful when you are altering the column specification of the table, because you do not need to change the code again.
NOTE: You can use also %type to declare variables and constants.
The General Syntax to declare a record of a uer-defined datatype is:
record_name record_type_name;
The following code shows how to declare a record called employee_rec based on a user-defined type.
DECLARE
TYPE employee_type IS RECORD
(employee_id number(5),
employee_first_name varchar2(25),
employee_last_name employee.last_name%type,
employee_dept employee.dept%type);
employee_salary employee.salary%type;
employee_rec employee_type;
If all the fields of a record are based on the columns of a table, we can declare the record as follows:
record_name table_name%ROWTYPE;
For example, the above declaration of employee_rec can as follows:
DECLARE
employee_rec employee%ROWTYPE;
The advantages of declaring the record as a ROWTYPE are:
1) You do not need to explicitly declare variables for all the columns in a table.
2) If you alter the column specification in the database table, you do not need to update the code.
The disadvantage of declaring the record as a ROWTYPE is:
1) When u create a record as a ROWTYPE, fields will be created for all the columns in the table and memory will be used to create the datatype for all the fields. So use ROWTYPE only when you are using all the columns of the table in the program.
NOTE: When you are creating a record, you are just creating a datatype, similar to creating a variable. You need to assign values to the record to use them.
The following table consolidates the different ways in which you can define and declare a pl/sql record.
Syntax
Usage
TYPE record_type_name IS RECORD (column_name1 datatype, column_name2 datatype, ...);
Define a composite datatype, where each field is scalar.
col_name table_name.column_name%type;
Dynamically define the datatype of a column based on a database column.
record_name record_type_name;
Declare a record based on a user-defined type.
record_name table_name%ROWTYPE;
Dynamically declare a record based on an entire row of a table. Each column in the table corresponds to a field in the record.
Passing Values To and From a Record
When you assign values to a record, you actually assign values to the fields within it.
The General Syntax to assign a value to a column within a record direclty is:
record_name.col_name := value;
If you used %ROWTYPE to declare a record, you can assign values as shown:
record_name.column_name := value;
We can assign values to records using SELECT Statements as shown:
SELECT col1, col2
INTO record_name.col_name1, record_name.col_name2
FROM table_name
[WHERE clause];
If %ROWTYPE is used to declare a record then you can directly assign values to the whole record instead of each columns separately. In this case, you must SELECT all the columns from the table into the record as shown:
SELECT * INTO record_name
FROM table_name
[WHERE clause];
Lets see how we can get values from a record.
The General Syntax to retrieve a value from a specific field into another variable is:
var_name := record_name.col_name;
The following table consolidates the different ways you can assign values to and from a record:
Syntax
Usage
record_name.col_name := value;
To directly assign a value to a specific column of a record.
record_name.column_name := value;
To directly assign a value to a specific column of a record, if the record is declared using %ROWTYPE.
SELECT col1, col2 INTO record_name.col_name1, record_name.col_name2 FROM table_name [WHERE clause];
To assign values to each field of a record from the database table.
SELECT * INTO record_name FROM table_name [WHERE clause];
To assign a value to all fields in the record from a database table.
variable_name := record_name.col_name;
To get a value from a record column and assigning it to a variable.
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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.
Error Handling: PL/SQL handles errors or exceptions effectively during the execution of a PL/SQL program. Once an exception is caught, specific actions can be taken depending upon the type of the exception or it can be displayed to the user with a message.
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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.
The PL/SQL Engine:
Oracle uses a PL/SQL engine to processes the PL/SQL statements. A PL/SQL code can be stored in the client system (client-side) or in the database (server-side).
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About This PL SQL Programming tutorial
This Oracle PL SQL tutorial teaches you the basics of programming in PL/SQL with appropriate examples. You can use this tutorial as your guide or reference while programming with PL SQL. I will be making this Oracle PL SQL programming tutorial as often as possible to share my knowledge in PL SQL and help you in learning PL SQL better.
Even though the programming concepts discussed in this tutorial is specific to Oracle PL SQL. The concepts like cursors, functions and stored procedures can be used in other database systems like Sybase , Microsoft SQL server etc, with some change in syntax. This tutorial will be growing regularly; let us know if any topic related to PL SQL needs to be added or you can also share your knowledge on PL SQL with us. Lets share our knowledge about PL SQL with others.
A Simple PL/SQL Block:
Each PL/SQL program consists of SQL and PL/SQL statements which from a PL/SQL block.
A PL/SQL Block consists of three sections:
The Declaration section (optional).
The Execution section (mandatory).
The Exception (or Error) Handling section (optional).
Declaration Section:
The Declaration section of a PL/SQL Block starts with the reserved keyword DECLARE. This section is optional and is used to declare any placeholders like variables, constants, records and cursors, which are used to manipulate data in the execution section. Placeholders may be any of Variables, Constants and Records, which stores data temporarily. Cursors are also declared in this section.
Execution Section:
The Execution section of a PL/SQL Block starts with the reserved keyword BEGIN and ends with END. This is a mandatory section and is the section where the program logic is written to perform any task. The programmatic constructs like loops, conditional statement and SQL statements form the part of execution section.
Exception Section:
The Exception section of a PL/SQL Block starts with the reserved keyword EXCEPTION. This section is optional. Any errors in the program can be handled in this section, so that the PL/SQL Blocks terminates gracefully. If the PL/SQL Block contains exceptions that cannot be handled, the Block terminates abruptly with errors.
Every statement in the above three sections must end with a semicolon ; . PL/SQL blocks can be nested within other PL/SQL blocks. Comments can be used to document code.
This is how a sample PL/SQL Block looks.
DECLARE
Variable declaration
BEGIN
Program Execution
EXCEPTION
Exception handling
END;
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