Procedures
Implementing Algorithms
SQL (Structured Query Language)
• SQL Structured Query Language is the standard way of accessing
Relational Database
• SQL consists of
• - DDL (Data Definition Language)
• - DML (Data Manipulation Language)
• SQL DML statements can be entered interactively
SQL …
• SQL commands are typed by user when they are to be executed
• Results are displayed in a
• - GUI (Graphical User Interface) or
• - CLI (Command Line Interface)
Applications
• Often lists of SQL commands must be run many times
• It would be difficult to type them in over and over
• Users can develop Applications built from Procedures
Procedures
• A procedure is the implementation of an algorithm.
• An algorithm is a list of instructions that perform a specific task
• The instructions are executed by a computer from top to bottom
• Instructions are executed from top to bottom
• The order can be changed by control statements
What is an algorithm?
Algorithm for driving a car:
• Get into the car
• Step on the break
• If standard, step on clutch
• Push “start” button
• Put car in gear
• Release break
• If standard, release clutch
Procedures and Functions
Procedure - A set of instructions implementing an algorithm.
Function - A procedure that returns a value.
Procedure format for most languages:
•
Procedure name (in some languages, not others)
•
Declarations
•
Executable instructions
Sample Procedure
REM Sample PL/SQL program
-- This program is kept in a file and run through the command or GUI interface
DECLARE
x INTEGER;
y INTEGER;
z INTEGER;
BEGIN
x := 1001;
y := x + 1000;
z := (x * 2) + 2000;
insert into xtab values (x, y, z);
END;
/
Comments
Entered when procedure is written
Make a program easy to understand
Read by person trying to understand the program
In SQL comment lines begin with “REM” or “--”
REM Sample PL/SQL program
-- This program is kept in a file and run
-- through the command or GUI interface
Declarations
• Many things can be declared for a procedure
• Varies from computer language to computer language
• Declarations tell the compiler or interpreter what objects will be
used in the procedure
Things that can be declared
Things that can be declared indifferent languages
•
Variables - named memory location to hold data
•
New Data Types - Usually based on language supported data types.
•
Special Variables - example SQL Cursors
•
Other Procedures and Functions
Constants and Variables
• A constant is a name given to a value.
• A variable is a named memory location that can hold data.
• In the quadratic formula AX^2 + BX + C
• A, B and C stand for constants
• X is a variable that can take on any value
How variables are used
• Variables can be declared and given values to hold
• Declare X Integer;
• Tells SQL that X is the name of a memory location that can hold integers.
Integers will be assigned to X below.
• X := 1001; -- Assigns causes the value 1001 to be stored in X
• X := X + 20; -- Will add 20 to what ever is in X and store the sum in X
SQL Statements in Procedures
• SQL statements can be placed in procedures
• They include: Select, Insert, Update, Delete
• SQL statements are slightly different in a procedure
• Involve variables as well as column names and constants
Sample Procedure
REM Sample PL/SQL program
-- This program is kept in a file and run through the command or GUI interface
DECLARE
x INTEGER;
y INTEGER;
z INTEGER;
BEGIN
x := 1001;
y := x + 1000;
z := (x * 2) + 2000;
insert into xtab values (x, y, z);
END;
/
Assignment Statement
• The Assignment Statement assigns a value to a variable
DECLARE
x INTEGER;
BEGIN
x := 1001;
:
-- This is a variable named “X”
-- Here the variable “X” is assigned the value 1001
Control Statements
• Control Statements alter the flow of instructions in the procedure
• They include the “IF-THEN-ELSE” and Looping instructions
• Order of instructions depends on a test performed by the control
statement.
IF-THEN-ELSE
• The IF-THEN-ELSE allows conditional execution
IF (Department = ‘Sales’) AND (Salary > $20000) THEN
INSERT INTO LOG TABLE VALUES (Name, Salary);
IF (Department = ‘Sales’) AND (Salary > $20000) THEN
INSERT INTO LOG TABLE VALUES (Name, Salary);
ELSE
UPDATE Stat_Table
Set LowVals = LowVals + 1
Where Department = ‘Sales’;
End If;
Looping Statements
• Looping statements allow a sequence of instructions to be repeated
X := 0;
LOOP
X := X + 1;
INSERT INTO LogTab Values (101, X);
IF X > 20 THEN
EXIT;
End IF;
END LOOP;
FOR LOOP
• FOR loop can be controlled to loop a given number of times.
FOR i in 1..3 LOOP
j := i*100;
k := j+1;
insert into xtab values (i, j, k);
END LOOP;
While Loop
• While loop will loop while a condition is true
I := 0;
WHILE I < 3 LOOP
j := i*100;
k := j+1;
insert into xtab values (i, j, k);
END LOOP;
END;
Implicit and Explicit Select
• SELECT statement can be used inside a procedure
• It will take data from the database to be used by the procedure
• Implicit SELECT limited to one row. Places data into variables
• Explicit SELECT puts many rows in buffer. Pulled into program by a
FETCH statement
Implicit SELECT statement
DECLARE
-- Variables to receive the values from the table
x INTEGER;
y INTEGER;
z INTEGER;
BEGIN
SELECT a, b, c INTO x, y, z -- In this statement a, b, and c are column names
FROM xtab
WHERE a = 101;
:
Explicit Select
The Explicit SELECT statement requires a cursor
You must declare a CURSOR with a SELECT statement
CURSOR c1 is
SELECT a, b, c from xtab;
You must OPEN the CURSOR to run the SELECT statement
Rows will be put in a buffer area
Open c1
Explicit SELECT continued
Use FETCH to access the rows in the buffer one at a time.
This is usually done in a loop
FETCH c1 INTO x, y, z;
Close the CURSOR when you are done
CLOSE c1;
Explicit Select continued
DECLARE
CURSOR c1 is
SELECT a, b, c from xtab;
x INTEGER;
y INTEGER;
z INTEGER;
BEGIN
OPEN c1;
FOR i in 1..3 LOOP
FETCH c1 INTO x, y, z;
INSERT into xtab values (x+10, y+10, z+10);
END LOOP;
CLOSE c1;
END;
Stored Procedures
• Stored Procedures are SQL procedures that are stored in the database
• Very similar to PS/SQL procedures
• They are created with a DDL statement
• They are run with an EXECUTE statement
Create Procedure
This statement creates a procedure but does not run it
Create Procedure ptab (x integer) as
begin
Update xtab
set c=c+100
where a>x;
end;
Execute Procedure
• This statement executes the procedure
execute ptab (150);
Procedures call procedures
Create Procedure ptab2 as
begin
ptab (150);
end;
execute ptab2;