introduction oracle plsql

7/30/2019 Introduction Oracle PLSQL

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Notes : Oracle PL/SQL - Intr oduction

Content

1.1.Basic Structure of PL/SQL 1.2. Variables and Types 1.3. Simple PL/SQL Programs 1.4. Control Flow in PL/SQL 1.5. Exception Handling 1.6. Cursor in COMPANY schema

1.1. Basic Str uctur e of PL/SQL

PL/SQL extends SQL by adding constructs found in procedural languages, resultingin a structural language that is more powerful than SQL. The basic unit in PL/SQL is

a block. The block containing executable SQL instructions is also called a script.

An example of a script is script for creating tables, inserting values and adding

constraints to existing tables. All these examples you can find in the part "creating

sample database".

Now we shall discuss PL/SQL blocks in more details.

The block consists of the following parts (not all of them are compulsory)

declare

{variable declarations}

begin {SQL and PL/SQL commands}

exception {series of exception handlers}

end;/

We illustrate first two parts on a simple example of computing area of a circle with

given radius. First we create two tables. Table AREAS contains two columns with

specification NUMBER(10,3) and names radius, areas. Table RADIUS_VALScontains just one column radius of the type NUMBER(10,3).

In the first example we insert into table AREAS one tuple corresponding to radius 3.

EXAMPLE:
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declarepi constant number(9,7) := 3.1415926;

/* constant value pi is declared with predefined value */

radius number (10,3);

area number (10,3);

/* variables radius and area are declared, without predefined values */

beginradius :=3;

area:= pi*power(radius,2);/* area of a circuit with radius 3 is computed */

insert into AREAS values (radius,area);

/* a tuple is inserted into a table

/* exception section in this block is missing */

end ;

This block has no name and it is possible to run it from SQL*plus prompt. It is

enough to have a valid account and create tables mentioned above. After executionthis script you can see following message:

PL/SQL procedure successfully completed.

When you write :

SELECT * FROM AREAS, The echo is following.

RADIUS AREA

---------- ----------

3 28.274

1 row selected.

1.2 Variables and Types

Information is transmitted between a PL/SQL program and the database through

variables. Every variable has a specific type associated with it. That type can be

One of the types used by SQL for database columns A generic type used in PL/SQL such as NUMBER Declared to be the same as the type of some database column


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The most commonly used generic type is NUMBER. Variables of type NUMBER can hold

either an integer or a real number. The most commonly used character string type is

VARCHAR(n), where n is the maximum length of the string in bytes. This length isrequired, and there is no default. For example, we might declare:DECLARE

FNAME VARCHAR(15);

DNO NUMBER;

Types in PL/SQL are tricky. Certain SQL types cannot be parameterized in PL/SQL.

For example, CHAR(50) does not work, but either CHAR or VARCHAR(50)works fine!

Also note that PL/SQL allows BOOLEAN variables, even though Oracle does not

support BOOLEAN as a type for database columns.

In many cases, a PL/SQL variable will be used to manipulate data stored in a existing

relation. In this case, it is essential that the variable have the same type as the relation

column. If there is any type mismatch, variable assignments and comparisons may not

work the way you expect. To be safe, instead of hard coding the type of a variable,

you should use the %TYPE operator. For example:

DECLARE

area AREAS.area %TYPE;

gives PL/SQL variable area the same type as was declared for the area column in thetable AREA..

A variable may also have a type that is a record with several fields. The simplest way

to declare such a variable is to use %ROWTYPE on a relation name. The result is a record

type in which the fields have the same names and types as the attributes of therelation. For instance:

DECLARE

AreaTuple AREAS %ROWTYPE;

makes variable AreaTuple be a record with fields: radius, area.

The initial value of any variable, regardless of its type, is NULL. We can assign values

to variables, using the ":=" operator. The assignment can occur either immediatelyafter the type of the variable is declared, or anywhere in the executable portion of the

program.

For reading values from existing tables we can use cursors. A cursor is declared in thedeclare section. In the following example we declare and use a cursor for reading

values of radius.


cursor rad_cursor is

select * from RADIUS_VALS;

/* definition of a cursor with name rad_cursor */

rad_val rad_cursor %ROWTYPE;

/* declaration of a variable rad_val with the same type as a record of table


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RADIUS_VALS


beginopen rad_cursor;

/* cursor rad_cursor is opened */

fetch rad_cursor into rad_val;/* one record (value) is fetched to record variable rad_val */area:= pi*power(rad_val.radius,2);

/* value rad_val.radius is used for computing the area */

insert into AREAS values(rad_val.radius,area);

close rad_cursor;

end;

In this example all records from the table RADIUS_VALS are read, but only one is

used for computing the area.

1.3 Simple Pr ograms in PL/SQL

In the PL/SQL programs we want repeat many times some group of operations. In

other words, we need loops. In the following example we present loop construction

for inserting values into table RADIUS_VALS.

declare

radius RADIUS_VALS.radius %TYPE;beginradius :=2;

/* starting value of the radius */

loop

/* beginning of a loop */insert into RADIUS_VALS values(radius);

radius:= radius+1;

exit when radius >10;

/* condition for exiting the loop

end loop;

end ;

In this example values from 2 to 10 are inserted to the table RADIUS_VALS.

Another example of the loop is a loop with cursor, where values of RADIUS_VALS

are read for producing tuples in the table AREAS.

declare

pi constant number(9,7) := 3.1415926;


select * from RADIUS_VALS;

rad_val rad_cursor %ROWTYPE;

area AREAS.area %TYPE;begin


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open rad_cursor;

loopfetch rad_cursor into rad_val;

exitwhen rad_cursor %NOTFOUND;

area:= pi*power(rad_val.radius,2);

insert into AREAS values(rad_val.radius,area);end loop;close rad_cursor;

end;

When run this script we obtain in the table AREAS values of areas of circles withradii r = 2 ...10.

Condition for exiting the loop is %NOTFOUND, it means no more records can be

fetched from the cursor. Another useful conditions related to cursor can be used:

%FOUND - A record can be fetched from the cursor%ISOPEN- The cursor have been opened%ROWCOUNT- the number of rows fetched from the cursor so far

1.4. Control Flow in PL/SQL

PL/SQL allows you to branch and create loops in a fairly familiar way.

An IF statement looks like:

if then else end if;

The ELSE part is optional. If you want a multiway branch, use:if then ...

elsif then ...

... ...

elsif then ...

else...

end if;

The following is an example, slightly modified from the previous one, where now we

only do the insertion if the area of the circuit is less then 100.

Example:

declare



select * from RADIUS_VALS;rad_val rad_cursor %ROWTYPE;


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beginopen rad_cursor;

loop

fetch rad_cursor into rad_val;

exit when rad_cursor %NOTFOUND;area:= pi*power(rad_val.radius,2);ifarea < 100 then

insert into AREAS values (rad_val.radius,area);

end if;

end loop;close rad_cursor;

end;

Loops are created with the following:

loop

/* A list of statements. */

end loop;

At least one of the statements in should be an EXIT statement of the

formexit when ;

The loop breaks if is true.

Some other useful loop-forming statements are:

exit by itself is an unconditional loop break. Use it inside a conditional if youlike.

A simple for loop can be formed with: for in .. loop

end loop;

Here, can be any variable; it is local to the for-loop and need not be

declared. Also, and are constants.

Example:

declare


area AREAS.area %TYPE;begin


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for radius in 1..7 loop

area:= pi*power(radius,2);

ifarea < 100 then

insert into AREAS values (radius,area);

end if;

end loop;end;

We can use for-loops together with cursors. In this case the cursor is opened andclosed automatically (without open, fetch and close instructions also %NOTFOUND

condition is not tested).

Example:


cursor rad_cursor isselect * from RADIUS_VALS;rad_val rad_cursor %ROWTYPE;


begin

for rad_val in rad_cursor looparea:= pi*power(rad_val.radius,2);

insert into AREAS values (rad_val.radius,area);

end loop;

end;

1.5. Exception Handling.

When user-defined or system-related errors are encountered, the control of PL/SQL

shifts to the Exception Handling section. Within the Exception section when clause is

used to evaluate which exception is to be handled. In our running example we define a

new variable some_var to illustrate an error than can happen during the execution of

our program. The error we expect is division by zero. In the case of division by zerothe default action is " insert (0,0) into the AREAS table. declare

pi constant number(9,7) := 3.1415926;radius number (10,3);

area number (10,3);

some_var number(10,3);

beginradius :=2;

loopsome_var := 1/(radius-5);

area:= pi*power(radius,2);

insert into AREAS values(radius,area);

radius:= radius+1;

exit when area >100;end loop;


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exceptionwhen ZERO_DIVIDE then

insert into AREAS values (0,0);

end ;

The action of the program is following. We compute values ofsome_varand area andinsert values of radius and area to the AREAS table. If there was not computing1/(radius -5 ) all values for area

1 declare /*Output variables*/

2 a EMPLOYEE.LNAME%TYPE;


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3 b EMPLOYEE.SSN%TYPE;

4 c EMPLOYEE.DNO%TYPE;

5 cursor EMPCursor IS

6 select LNAME,SSN,DNO

7 from EMPLOYEE;

8 begin

9 open EMPCursor;10 loop

11 fetch EMPCursor into a,b,c;

12 exit when EMPCursor%NOTFOUND;

13 if C>=4 THEN INSERT into EMPLOYEE1 values (a,b,c);

14 end if;

15 end loop;

16 close EMPCursor;

17* end;

Here are explanations for the various lines of this program:

Line (1) introduces the declaration section. Lines (2),(3) and (4) declare variables a,b and c to have types equal to the

types of attributes of the relation Employee1.

Lines (5) through (7) define the cursor EMPCursor. It ranges over a relationdefined by the SELECT-FROM-WHERE query. That query selects all tuples of

Employee.

Line (8) begins the executable section of the program. Line (9) opens the cursor, an essential step. Lines (10) through (15) are a PL/SQL loop. Notice that such a loop is

bracketed by LOOP and END LOOP. Within the loop we find:

o On Line (11), a fetch through the cursor into the local variables. Ingeneral, the

FETCHstatement must provide variables for eachcomponent of the tuple retrieved. Since the query of Lines (5) through

(7) produces triples, we have correctly provided three variables, and

we know they are of the correct type.

o On Line (12), a test for the loop-breaking condition. Its meaningshould be clear: %NOTFOUND after the name of a cursor is true exactly

when a fetch through that cursor has failed to find any more tuples.

o On Line (13), a SQL INSERT statement that inserts the tuple intoEmployee1.

Line (16) closes the cursor. Line (17) ends the PL/SQL program.

It is possible of course to copy the above tuples into the table Employee1 bystandalone SQL construction. We encourage the reader to try it and to compare the

result.

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