PL/SQL Tutorial

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).

A Simple PL/SQL Block:

Each PL/SQL program consists of SQL and PL/SQL statements which from a PL/SQL block.

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.

How a Sample PL/SQL Block Looks

DECLARE      Variable declaration BEGIN      Program Execution EXCEPTION      Exception handling END;

PL/SQL Placeholders

Placeholders are temporary storage area. Placeholders can be any of Variables, Constants and Records. Oracle defines placeholders to store data temporarily, which are used to manipulate data during the execution of a PL SQL block.

Depending on the kind of data you want to store, you can define placeholders with a name and a datatype. Few of the datatypes used to define placeholders are as given below.
Number (n,m) , Char (n) , Varchar2 (n) , Date , Long , Long raw, Raw, Blob, Clob, Nclob, Bfile

PL/SQL Variables

These are placeholders that store the values that can change through the PL/SQL Block.

General Syntax to declare a variable is variable_name datatype [NOT NULL := value ]; variable_name is the name of the variable. datatype is a valid PL/SQL datatype. NOT NULL is an optional specification on the variable. value or DEFAULT valueis also an optional specification, where you can initialize a variable. Each variable declaration is a separate statement and must be terminated by a semicolon. For example, if you want to store the current salary of an employee, you can use a variable. DECLARE salary  number (6); * “salary” is a variable of datatype number and of length 6. When a variable is specified as NOT NULL, you must initialize the variable when it is declared.

For example: The below example declares two variables, one of which is a not null.

DECLARE

salary number(4);

dept varchar2(10) NOT NULL := “HR Dept”;

The value of a variable can change in the execution or exception section of the PL/SQL Block. We can assign values to variables in the two ways given below.
1) We can directly assign values to variables.
    The General Syntax is:         

  variable_name:=  value;

2) We can assign values to variables directly from the database columns by using a SELECT.. INTO statement. The General Syntax is:

SELECT column_name INTO variable_name FROM table_name [WHERE condition];

Example: The below program will get the salary of an employee with id '1116' and display it on the screen.

DECLARE 

 var_salary number(6); 

 var_emp_id number(6) = 1116; 

BEGIN

 SELECT salary 

 INTO var_salary 

 FROM employee 

 WHERE emp_id = var_emp_id; 

 dbms_output.put_line(var_salary); 

 dbms_output.put_line('The employee ' 

        || var_emp_id || ' has  salary  ' || var_salary); 

END; 

/

NOTE: The backward slash '/' in the above program indicates to execute the above PL/SQL Block.

Scope of PS/SQL Variables

PL/SQL allows the nesting of Blocks within Blocks i.e, the Execution section of an outer block can contain inner blocks. Therefore, a variable which is accessible to an outer Block is also accessible to all nested inner Blocks. The variables declared in the inner blocks are not accessible to outer blocks. Based on their declaration we can classify variables into two types.

• Local variables - These are declared in a inner block and cannot be referenced by outside Blocks.
• Global variables - These are declared in a outer block and can be referenced by its itself and by its inner blocks.

For Example: In the below example we are creating two variables in the outer block and assigning thier product to the third variable created in the inner block. The variable 'var_mult' is declared in the inner block, so cannot be accessed in the outer block i.e. it cannot be accessed after line 11. The variables 'var_num1' and 'var_num2' can be accessed anywhere in the block.

1> DECLARE 2>  var_num1 number; 3>  var_num2 number; 4> BEGIN 5>  var_num1 := 100; 6>  var_num2 := 200; 7>  DECLARE 8>   var_mult number; 9>   BEGIN 10>    var_mult := var_num1 * var_num2; 11>   END; 12> END; 13> /    PL/SQL Constants As the name implies a constant is a value used in a PL/SQL Block that remains unchanged throughout the program. A constant is a user-defined literal value. You can declare a constant and use it instead of actual value. For example: If you want to write a program which will increase the salary of the employees by 25%, you can declare a constant and use it throughout the program. Next time when you want to increase the salary again you can change the value of the constant which will be easier than changing the actual value throughout the program. General Syntax to declare a constant is: constant_name CONSTANT datatype := VALUE; constant_name is the name of the constant i.e. similar to a variable name. The word CONSTANT is a reserved word and ensures that the value does not change. VALUE - It is a value which must be assigned to a constant when it is declared. You cannot assign a value later. For example, to declare salary_increase, you can write code as follows: DECLARE salary_increase CONSTANT number (3) := 10; You must assign a value to a constant at the time you declare it. If you do not assign a value to a constant while declaring it and try to assign a value in the execution section, you will get a error. If you execute the below Pl/SQL block you will get error. DECLARE  salary_increase CONSTANT number(3); BEGIN  salary_increase := 100;  dbms_output.put_line (salary_increase); 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.   Conditional Statements in PL/SQL As the name implies, PL/SQL supports programming language features like conditional statements, iterative statements. The programming constructs are similar to how you use in programming languages like Java and C++. In this section I will provide you syntax of how to use conditional statements in PL/SQL programming. IF THEN ELSE STATEMENT 1) IF condition THEN  statement 1; ELSE  statement 2; END IF;   2) IF condition 1 THEN  statement 1;  statement 2; ELSIF condtion2 THEN  statement 3; ELSE  statement 4; END IF 3) IF condition 1 THEN  statement 1;  statement 2; ELSIF condtion2 THEN  statement 3; ELSE  statement 4; END IF;   4) IF condition1 THEN ELSE  IF condition2 THEN  statement1;  END IF; ELSIF condition3 THEN   statement2; END IF; Iterative Statements in PL/SQL Iterative control Statements are used when we want to repeat the execution of one or more statements for specified number of times. There are three types of loops in PL/SQL: • Simple Loop • While Loop • For Loop 1) Simple Loop A Simple Loop is used when a set of statements is to be executed at least once before the loop terminates. An EXIT condition must be specified in the loop, otherwise the loop will get into an infinite number of iterations. When the EXIT condition is satisfied the process exits from the loop.   General Syntax to write a Simple Loop is: LOOP    statements;    EXIT;    {or EXIT WHEN condition;} END LOOP; These are the important steps to be followed while using Simple Loop. 1) Initialise a variable before the loop body. 2) Increment the variable in the loop. 3) Use a EXIT WHEN statement to exit from the Loop. If you use a EXIT statement without WHEN condition, the statements in the loop is executed only once. 2) While Loop A WHILE LOOP is used when a set of statements has to be executed as long as a condition is true. The condition is evaluated at the beginning of each iteration. The iteration continues until the condition becomes false. The General Syntax to write a WHILE LOOP is: WHILE <condition>  LOOP statements; END LOOP; Important steps to follow when executing a while loop: 1) Initialise a variable before the loop body. 2) Increment the variable in the loop. 3) EXIT WHEN statement and EXIT statements can be used in while loops but it's not done oftenly. 3) FOR Loop A FOR LOOP is used to execute a set of statements for a predetermined number of times. Iteration occurs between the start and end integer values given. The counter is always incremented by 1. The loop exits when the counter reachs the value of the end integer. The General Syntax to write a FOR LOOP is: FOR counter IN val1..val2   LOOP statements; END LOOP; val1 - Start integer value. val2 - End integer value. Important steps to follow when executing a while loop: 1) The counter variable is implicitly declared in the declaration section, so it's not necessary to declare it explicity. 2) The counter variable is incremented by 1 and does not need to be incremented explicitly. 3) EXIT WHEN statement and EXIT statements can be used in FOR loops but it's not done oftenly. NOTE: The above Loops are explained with a example when dealing with Explicit Cursors. 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. The 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 For Example: Consider the PL/SQL Block that uses implicit cursor attributes as shown below: DECLARE  var_rows number(5); BEGIN   UPDATE employee   SET salary = salary + 1000;   IF SQL%NOTFOUND THEN     dbms_output.put_line('None of the salaries where updated');   ELSIF SQL%FOUND THEN     var_rows := SQL%ROWCOUNT;     dbms_output.put_line('Salaries for ' || var_rows || 'employees are updated');   END IF; END; In the above PL/SQL Block, the salaries of all the employees in the ‘employee’ table are updated. If none of the employee’s salary are updated we get a message 'None of the salaries where updated'. Else we get a message like for example, 'Salaries for 1000 employees are updated' if there are 1000 rows in ‘employee’ table. Explicit Cursors An explicit cursor is defined in the declaration section of the PL/SQL Block. It is created on a SELECT Statement which returns more than one row. We can provide a suitable name for the cursor. General Syntax for creating a cursor is as given below: CURSOR cursor_name IS select_statement; cursor_name – A suitable name for the cursor. select_statement – A select query which returns multiple rows. How to use Explicit Cursor? There are four steps in using an Explicit Cursor. DECLARE the cursor in the declaration section. OPEN the cursor in the Execution Section. FETCH the data from cursor into PL/SQL variables or records in the Execution Section. CLOSE the cursor in the Execution Section before you end the PL/SQL Block. 1) Declaring a Cursor in the Declaration Section:    DECLARE    CURSOR emp_cur IS    SELECT *    FROM emp_tbl    WHERE salary > 5000;       In the above example we are creating a cursor ‘emp_cur’ on a query which returns the records of all the employees with salary greater than 5000. Here ‘emp_tbl’ in the table which contains records of all the employees. 2) Accessing the records in the cursor:       Once the cursor is created in the declaration section we can access the cursor in the execution       section of the PL/SQL program. How to access an Explicit Cursor? These are the three steps in accessing the cursor. 1) Open the cursor. 2) Fetch the records in the cursor one at a time. 3) Close the cursor. General Syntax to open a cursor is: OPEN cursor_name;   General Syntax to fetch records from a cursor is: FETCH cursor_name INTO record_name; OR FETCH cursor_name INTO variable_list; General Syntax to close a cursor is: CLOSE cursor_name; When a cursor is opened, the first row becomes the current row. When the data is fetched it is copied to the record or variables and the logical pointer moves to the next row and it becomes the current row. On every fetch statement, the pointer moves to the next row. If you want to fetch after the last row, the program will throw an error. When there is more than one row in a cursor we can use loops along with explicit cursor attributes to fetch all the records. Points to remember while fetching a row: · We can fetch the rows in a cursor to a PL/SQL Record or a list of variables created in the PL/SQL Block. · If you are fetching a cursor to a PL/SQL Record, the record should have the same structure as the cursor. · If you are fetching a cursor to a list of variables, the variables should be listed in the same order in the fetch statement as the columns are present in the cursor. General Form of using an explicit cursor is:  DECLARE     variables;     records;     create a cursor; BEGIN    OPEN cursor;    FETCH cursor;      process the records;    CLOSE cursor; END; Explicit Cursor, Lets Look at the example below Example 1: 1> DECLARE 2>    emp_rec emp_tbl%rowtype; 3>    CURSOR emp_cur IS 4>    SELECT * 5>    FROM 6>    WHERE salary > 10; 7> BEGIN 8>    OPEN emp_cur; 9>    FETCH emp_cur INTO emp_rec; 10>      dbms_output.put_line (emp_rec.first_name || '  ' || emp_rec.last_name); 11>   CLOSE emp_cur; 12> END; In the above example, first we are creating a record ‘emp_rec’ of the same structure as of table ‘emp_tbl’ in line no 2. We can also create a record with a cursor by replacing the table name with the cursor name. Second, we are declaring a cursor ‘emp_cur’ from a select query in line no 3 - 6. Third, we are opening the cursor in the execution section in line no 8. Fourth, we are fetching the cursor to the record in line no 9. Fifth, we are displaying the first_name and last_name of the employee in the record emp_rec in line no 10. Sixth, we are closing the cursor in line no 11. What are Explicit Cursor Attributes? Oracle provides some attributes known as Explicit Cursor Attributes to control the data processing while using cursors. We use these attributes to avoid errors while accessing cursors through OPEN, FETCH and CLOSE Statements. When does an error occur while accessing an explicit cursor?  a) When we try to open a cursor which is not closed in the previous operation. b) When we try to fetch a cursor after the last operation. These are the attributes available to check the status of an explicit cursor. Attributes Return values Example %FOUND TRUE, if fetch statement returns at least one row. Cursor_name%FOUND FALSE, if fetch statement doesn’t return a row. %NOTFOUND TRUE, , if fetch statement doesn’t return a row. Cursor_name%NOTFOUND FALSE, if fetch statement returns at least one row. %ROWCOUNT The number of rows fetched by the fetch statement Cursor_name%ROWCOUNT If no row is returned, the PL/SQL statement returns an error. %ISOPEN TRUE, if the cursor is already open in the program Cursor_name%ISNAME FALSE, if the cursor is not opened in the program. Using Loops with Explicit Cursors: Oracle provides three types of cursors namely SIMPLE LOOP, WHILE LOOP and FOR LOOP. These loops can be used to process multiple rows in the cursor. Here I will modify the same example for each loops to explain how to use loops with cursors. Cursor with a Simple Loop: 1> DECLARE 2>   CURSOR emp_cur IS 3>   SELECT first_name, last_name, salary FROM emp_tbl; 4>   emp_rec emp_cur%rowtype; 5> BEGIN 6>   IF NOT sales_cur%ISOPEN THEN 7>      OPEN sales_cur; 8>   END IF; 9>   LOOP 10>     FETCH emp_cur INTO emp_rec; 11>     EXIT WHEN emp_cur%NOTFOUND; 12>     dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name 13>     || ' ' ||emp_cur.salary); 14>  END LOOP; 15>  END; 16>  / In the above example we are using two cursor attributes %ISOPEN and %NOTFOUND. In line no 6, we are using the cursor attribute %ISOPEN to check if the cursor is open, if the condition is true the program does not open the cursor again, it directly moves to line no 9. In line no 11, we are using the cursor attribute %NOTFOUND to check whether the fetch returned any row. If there is no rows found the program would exit, a condition which exists when you fetch the cursor after the last row, if there is a row found the program continues. We can use %FOUND in place of %NOTFOUND and vice versa. If we do so, we need to reverse the logic of the program. So use these attributes in appropriate instances. Cursor with a While Loop: Lets modify the above program to use while loop. 1> DECLARE 2>  CURSOR emp_cur IS 3>  SELECT first_name, last_name, salary FROM emp_tbl; 4>  emp_rec emp_cur%rowtype; 5> BEGIN 6>   IF NOT sales_cur%ISOPEN THEN 7>      OPEN sales_cur; 8>   END IF; 9>   FETCH sales_cur INTO sales_rec;  10>  WHILE sales_cur%FOUND THEN  11>  LOOP 12>    dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name 13>    || ' ' ||emp_cur.salary); 15>    FETCH sales_cur INTO sales_rec; 16>  END LOOP; 17> END; 18> / In the above example, in line no 10 we are using %FOUND to evaluate if the first fetch statement in line no 9 returned a row, if true the program moves into the while loop. In the loop we use fetch statement again (line no 15) to process the next row. If the fetch statement is not executed once before the while loop the while condition will return false in the first instance and the while loop is skipped. In the loop, before fetching the record again, always process the record retrieved by the first fetch statement, else you will skip the first row. Cursor with a FOR Loop: When using FOR LOOP you need not declare a record or variables to store the cursor values, need not open, fetch and close the cursor. These functions are accomplished by the FOR LOOP automatically. General Syntax for using FOR LOOP: FOR record_name IN cusror_name LOOP     process the row... END LOOP; Let’s use the above example to learn how to use for loops in cursors. 1> DECLARE 2>  CURSOR emp_cur IS 3>  SELECT first_name, last_name, salary FROM emp_tbl; 4>  emp_rec emp_cur%rowtype; 5> BEGIN 6>  FOR emp_rec in sales_cur 7>  LOOP  8>  dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name 9>    || ' ' ||emp_cur.salary);  10> END LOOP; 11>END; 12> / In the above example, when the FOR loop is processed a record ‘emp_rec’of structure ‘emp_cur’ gets created, the cursor is opened, the rows are fetched to the record ‘emp_rec’ and the cursor is closed after the last row is processed. By using FOR Loop in your program, you can reduce the number of lines in the program. 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.