Introduction to PL/SQL

Introduction to PL/SQL
Sen Zhang

Fundamentals This lecture serves as an introduction to PL/SQL –
a powerful programming language that works hand in hand with SQL.

Objectives Learn the fundamentals of the PL/SQL programming language
How to write and execute PL/SQL programs in SQL*Plus Understand PL/SQL data type conversion functions Manipulate character strings in PL/SQL programs Learn how to debug PL/SQL programs

SQL vs. PL/SQL As we have learned, one benefit of SQL is that it is declarative, allowing us to easily create Oracle database tables and write queries to insert, update, delete, and view records without specifying too much detailed data manipulation steps. There is no free lunch though. The weakness for SQL is also due to its declarative language feature: it has no support for procedural constructs such as flow control, or loop mechanisms, etc. While a single SQL command usually cannot satisfy Non-trivial business logics; complex business logics demand more powerful programming scheme like procedural language.

Procedural vs. declarative
Procedural programming requires that the programmer tells how to process data using detailed sequential or flow control instructions to. Bad: Procedural languages result in many lines of code. Good: Have more control on detail Declarative programming requires the programmer to tell what data is needed. Good: The Declarative languages usually result in one statement of the desired result. Bad: Have less control on data.

SQL is not a procedural language but a declarative language.
Using SQL, we carefully phrase what we want and then let the DBMS get it for us. You write a single SQL declaration and hand it to the DBMS. The DBMS then executes internal code, which is hidden from us.

Why SQL is declarative not procedural?
SQL came into existence as a relational database query language. SQL was designed, evolved, optimized for structured data, i.e. table, manipulation, saving people from considering tedious step by step instructions. Being declarative is exactly what we want when it comes to data manipulation. Not optimized nor designed for logics. This is a problem!!!

An example. Let us look at the a simplified business logic (business rule) involved in creating a new user account, which is useful in any user restricted database application. Assuming we have a user table to store account infor Three inputs: username, password, and password confirmation Verify two passwords, if not equal, report failure reason Otherwise, verify whether username is new or not. If username exists, the new account cannot be added to the user table; report failure reason. If username does not exist, go ahead to insert the new record to table; report success status.

Solution Here, at least, variables, conditional logic and branch action are involved, what will happen is conditioned by user input and the current database status. We, as programmers, can take the responsibility to coordinate the logic, can manually type several sql statements to get the job done.

End user vs. programmer But the terminal user is not necessarily a programmer. Most database users, reasonably assumed to have no programming knowledge at all, don’t use SQL commands or SQLPLUS to interact with a database. Instead, users use GUI frontend interface to deal with backend database. The terminal user Is God! They just want to type into text boxes, and point and click buttons and …. Wait output to show up. In run time, the end users provide input data and interpret output, but the logic needs to be implemented in the design time in the program which should have been done by programmers.

This means we have to implement this logic in a program, using some kind of procedural programming language, not just SQL. Since database application is so important, other procedural programming languages have been designed to be able to contain SQL commands and interact with an Oracle database. We can write applications with SQL statements embedded within a host language such as C++, Java, vb or vbscript etc.. The SQL statements provide the database interface, while the host language provides the remaining support needed for the application to execute.

We can take a lot at VB script solution in an asp page using ODBC and ADODB
Java can also be used to talk to Oracle through JDBC.

PL/SQL PL/SQL is the Oracle solution to this need,
PL/SQL stands for Procedural Language/SQL. PL/SQL extends SQL by adding constructs found in procedural languages, resulting in a structural language that is more powerful than SQL. A procedural programming language that pure oracle applications uses to manipulate database data. A complement to SQL. An extension to SQL, allowing us do things we cannot do in SQL alone.

Good things combined A PL/SQL program combines good things from both sides, combining SQL quries with procedural commands for tasks such as manipulating variable values, Evaluating IF/THEN decision control structures, Creating loop structures that repeat instructions multiple times until the loop reaches an exit condition. Build in functions User defined functions Encapsulations using packages

A Full-featured procedural programming language
An interpreted language, which means that a program called the PL/SQL interpreter checks each program command for syntax errors, translates each command into machine language, and then executes each program command, one command at a time. PL/SQL commands are not case-sensitive, except for character strings, which you must enclose in single quotation marks. The PL/SQL interpreter ignores blank spaces and line breaks. A semicolon (;) marks the end of each PL/SQL command. The programming style is a mixture of conventional statements (if, while, etc.) and SQL statements.

What are the benefits PL/SQL brings to us?
Using PL/SQL, you can use various regular procedural programming language constructs to implement complex business logics. You can also develop stored procedures, functions, and triggers easily and in a modular fashion. We are used to stored data, but not stored commands. Yes! Here, we will be able to make procedures stored in the database. The stored components are stored directly in the database, which makes the program available to all database users if the access privileges has been properly granted. This stored feature also makes it easier to manage database applications, providing a unified API interface to different front end applications and different front end languages.

Stored? It means surviving the sessions or connections.
Stored on server permenently.

How to edit PL/SQL programs
Type in any text editor, submitted to Oracle server through SQL*Plus

Comments Enclosed between /* and */ Not executed by interpreter
C style comments (/* ... */) may be used. Enclosed between /* and */ On one line beginning with --

PL/SQL is a programming language or procedural language
Since we are talking about a procedural language, we are interested in various language constructs which constitute a procedural language. Reserved word Data type Variable Statement Arithmetic operation Logical operation Control constructs Loop Branch block Built in functions How to define user defined functions How to call user defined functions …

PL/SQL is not case sensitive.

Reserved word Each of the reserved words has a special syntactic meaning to PL/SQL. So, you should not use them to name program objects such as constants, variables, or cursors. Also, some of these words (marked by an asterisk) are reserved by SQL. So, you should not use them to name database objects such as columns, tables, or indexes.

Variables and Data Types
Information is transmitted between a PL/SQL program and the database through variables. Variables need to be declared!

Data types PL/SQL is a strongly typed language, which means that you must write a command that explicitly declares each variable and specifies its data type before you use the variable. With a strongly typed language, you can assign values to variables and compare variable values only for variables with the same data type or compatible data type.

Every PL/SQL variable has a specific type associated with it.
There are four kinds of data types Scalar data type One of the types used by SQL for database columns A generic type used in PL/SQL such as NUMBER Composite data type Reference data type Declared to be the same as the type of some database column LOB data types for huge binary data used by images and sounds

Scalar Data Types Represent a single value
Description Sample Declaration Varchar2 Variable-length character string Lastname varchar2(30) char Fixed-length character string Gender char(1) Number Floating-point, fixed-point, or integer number Price number(5,2) date Date and time Todays_date Date; …. These data types are directly from data types used by SQL database field specification.

Bridging role reflected in data types
Some other data types used PL/SQL are more general purpose programming language oriented, not corresponding to database data types. INTEGER BOOLEAN DECIMAL … Note that PL/SQL allows BOOLEAN variables, even though Oracle does not support BOOLEAN as a type for database columns.

Composite data types Composite data types
RECORD: contains multiple scalar values, similar to a table record TABLE: tabular structure with multiple columns and rows VARRAY: variable-sized array

Reference data types %TYPE: same data type as a database field
In many cases, a PL/SQL variable will be used to manipulate data stored in a existing table. In this case, it is essential that the variable have the same type (compatible is also ok in some situation) as the relation column. Directly reference a specific database field or record and assume the data type of the associated field or record %TYPE: same data type as a database field %ROWTYPE: same data type as a database record

Data Types in PL/SQL can be tricky!
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 myBeer Beers.name%TYPE; gives PL/SQL variable myBeer whatever type was declared for the name column in relation Beers

Variables Userid varchar2(10); Variables
Used to store numbers, character strings, dates, and other data values Avoid using keywords, table names and column names as variable names Must be declared with data type before use: variable_name data_type_declaration; Userid varchar2(10); Default value is always NULL when declared without being initialized. The initial value of any variable, regardless of its type, is NULL.

Variable names are any valid PL/SQL identifiers.
Read book page 300 for what a valid identifier is.

Remarks Data types in a procedure definition specification cannot have size specifications. For instance, you can specify that a parameter is a NUMBER datatype, but not a NUMBER(10,2)

Assignment Statements
We can assign values to variables, using the ":=" operator. Like any other programming languages you might have used before, the assignment can occur either immediately after the type of the variable is declared, or anywhere in the executable portion of the program. Assigns a value to a variable variable_name := value; Value can be a literal: s_first_name := ‘Steven'; Value can be another variable: first_name := s_first_name;

How to link variables between SQL command and PL command?
We will solve this problem in sample code.

Some simple statements
Return Goto <label> Exit, break a loop

Arithmetic Operators in PL/SQL
Description ** Exponentiation * Multiplication / Division + and - Addition and subtraction - negation

Most SQL Relational Operators can be used for PL/SQL
Query Relational Operators Operator Meaning Data Type Pattern = equal to all =X > greater than >X < less than <X >= greater than or equal to >=X <= less than or equal to <=X <> or != not equal to <>X * wildcard Character *X, X*, *X* ? single-character wildcard ?X, X?, ?X?, X??

Logical Operators and, or, not

Expressions Simple arithmetic expressions
Simple relational expressions Simple logical expressions Nested and compound expressions

Built-in functions You can also you built-in functions to perform common tasks such as manipulating numbers or character strings.

PL/SQL Data Conversion Functions

Manipulating Character Strings with PL/SQL
To concatenate two strings in PL/SQL, you use the double bar (||) operator: new_string := string1 || string2; To remove blank leading spaces use the LTRIM function: string := LTRIM(string_variable_name); To remove blank trailing spaces use the RTRIM function: string := RTRIM(string_variable_name); To find the number of characters in a character string use the LENGTH function: string_length := LENGTH(string_variable_name);

Manipulating Character Strings with PL/SQL
To change case, use UPPER, LOWER, INITCAP INSTR function searches a string for a specific substring: start_position := INSTR(original_string, substring); SUBSTR function extracts a specific number of characters from a character string, starting at a given point: extracted_string := SUBSTR(string_variable, starting_point, number_of_characters);

The basic unit in PL/SQL is a block.
All PL/SQL programs are made up of blocks, which can be nested within each other. Typically, each block performs a logical action in he program. A block has the following structure: DECLARE /* Declarative section: variables, types, and local subprograms. */ BEGIN /* Executable section: procedural and SQL statements go here. */ /* This is the only section of the block that is required. */ EXCEPTION /* Exception handling section: error handling statements go here. */ END;

A PL/SQL block contains 1 or more PL/SQL statements
A PL/SQL block contains 1 or more PL/SQL statements. Such a block must at least have the two keywords begin and end: begin PL contaminated SQL statements The executable section also contains constructs such as assignments, branches, loops, procedure calls, and …. end; I call them PL contaminated SQL statements, because they are not pure SQL, they contains variables of PL part. Sometimes, not PL contaminated for trivial operations.

The only SQL statements allowed in a PL/SQL program are SELECT, INSERT, UPDATE, DELETE and several other data manipulation statements plus some transaction control. For most cases, they have to be PL contaminated though! Data definition statements like CREATE, DROP, or ALTER are not allowed.

A PL/SQL block The header section Declaration section, optional
Execution section Optional exception section

The major nuance is that the form of the SELECT statement is different from its SQL form. After the SELECT clause, we must have an INTO clause listing variables, one for each attribute in the SELECT clause, into which the components of the retrieved tuple must be placed. Notice we said "tuple" rather than "tuples", since the SELECT statement in PL/SQL only works if the result of the query contains a single tuple. If the query returns more than one tuple, you need to use a cursor,!!!! as described in the future lectures.

PL/SQL Program Blocks

The scope of A PL/SQL block for local variables
A PL/SQL block establishes a scope for all locally-declared variables. Outside of the block, those variables do not exist.

Executing a PL/SQL Program in SQL*Plus
The built-in function SYSDATE returns a DATE value containing the current date and time on your system.

How to test out the PL/SQL program?
Create program in text editor Paste into SQL*Plus window Press Enter, type / then enter to execute. The forward slash (/) tells SQL*PLUS to go ahead and process the commands in the program.

Debugging PL/SQL Programs
Syntax error: Command does not follow the guidelines of the programming language Generates compiler or interpreter error messages Logic error: Program runs but results in an incorrect result Caused by mistakes at semantic level in programing

Finding and Fixing Syntax Errors
Interpreter flags the line number and character location of syntax errors If error message appears and the flagged line appears correct, the error usually occurs on program lines preceding the flagged line Comment out program lines to look for hidden errors One error (such as missing semicolon) may cause more – fix one error at a time

Finding and Fixing Logic Errors
Locate logic errors by viewing variable values during program execution There is no SQL*Plus debugger Use DBMS_OUTPUT statements to print variable values

Objectives Create PL/SQL decision control structures
Use SQL queries in PL/SQL programs Create loops in PL/SQL programs Create PL/SQL tables and tables of records Use cursors to retrieve database data into PL/SQL programs Use the exception section to handle errors in PL/SQL programs

PL/SQL Control Structures
PL/SQL allows you to branch and create loops and function calls in the way that you have been doing in C++/Java. If statement Loops: three different iteration constructs. Loop While For ..

PL/SQL Decision Control Structures
One way branch! We also say the commands are conditioned. Use IF/THEN structure to execute code if condition is true. IF condition THEN commands that execute if condition is TRUE; END IF; If condition evaluates to NULL it is considered false

Two way decision Use IF/THEN/ELSE to execute code if condition is true or false IF condition THEN commands that execute if condition is TRUE; ELSE commands that execute if condition is FALSE; END IF; Can be nested – be sure to end nested statements

PL/SQL Decision Control Structures
Multi-way branches Use IF/ELSIF to evaluate many conditions: IF condition1 THEN commands that execute if condition1 is TRUE; ELSIF condition2 THEN commands that execute if condition2 is TRUE; ELSIF condition3 THEN commands that execute if condition3 is TRUE; ... ELSE commands that execute if none of the conditions are TRUE; END IF;

IF/ELSIF Example

Complex Conditions Created with logical operators AND, OR and NOT
AND is evaluated before OR Use () to set precedence

MIX SQL and PL/SQL DDL commands may not be used in PL/SQL (usually we do not do that anyway.)

Using SQL Queries in PL/SQL Programs (This is where SQL and PL/SQL communicate.)
Insert, delete and update statements (we also call them action queries, because these commands perform an action that changes the data values in the database.) can be used as in SQL*Plus Usually use variables in action queries

Using SQL Queries in PL/SQL Programs
Select command can be used, but how to use them usually depending on what will be returned from select command. Single row, then use into assign the retrieved values to variables. Then you can manipulate the values in program commands. Multiple row, you want to use cursor.

Using an Implicit Cursor (Implicit Cursor will be discussed!)
Executing a SELECT query creates an implicit cursor To retrieve it into a variable use INTO: SELECT field1, field2, ... INTO variable1, variable2, ... FROM table1, table2, ... WHERE join_ conditions AND search_condition_to_retrieve_1_record; Can only be used with queries that return exactly one record

The select statement that finds the total number of employee is a simple select statement with added keyword INTO. The INTO part of the statement is required in order to put the values returned by the select statement into the corresponding PL/SQL variables. If it returns more than one record, obviously, there will something wrong. To address this issue, we usually need to use explicit cursor.

Loops Program structure that executes a series of program statements, and periodically evaluates an exit condition to determine if the loop should repeat or exit Pretest loop: evaluates the exit condition before any program commands execute Posttest loop: executes one or more program commands before the loop evaluates the exit condition for the first time PL/SQL has 5 loop structures

The LOOP...EXIT Loop LOOP [program statements] IF condition THEN EXIT;
END IF; [additional program statements] END LOOP

The LOOP...EXIT WHEN Loop LOOP program statements EXIT WHEN condition;
END LOOP;

The WHILE...LOOP WHILE condition LOOP program statements END LOOP;

The Numeric FOR Loop FOR counter_variable IN start_value .. end_value
program statements END LOOP;

Cursors Pointer to a memory location that the DBMS uses to process a SQL query Use to retrieve and manipulate database data

When you execute a SQL statement from PL/SQL, the Oracle RDBMS assigns a private work area for that statement. This work area contains information about the SQL statement and the set of data returned or affected by that statement. The PL/SQL cursor is a mechanism by which you can name that work area and manipulate the information within it.

Implicit Cursor

Using an Implicit Cursor
Executing a SELECT query creates an implicit cursor To retrieve it into a variable use INTO: SELECT field1, field2, ... INTO variable1, variable2, ... FROM table1, table2, ... WHERE join_ conditions AND search_condition_to_retrieve_1_record; Can only be used with queries that return exactly one record

The select statement that finds the total number of employee is a simple select statement with added keyword INTO. The INTO part of the statement is required in order to put the values returned by the select statement into the corresponding PL/SQL variables. If it returns more than one record, obviously, there will something wrong.

Explicit Cursor Use for queries that return multiple records or no records Must be explicitly declared and used

Cursor CURSOR employee_cur IS SELECT * FROM employee;
In its simplest form, you can think of a cursor as a pointer into a relation in the database or dynamically generated from other relations. For example, the following cursor declaration associates the entire employee table with the cursor named employee_cur: Step 1: cursor declaration CURSOR employee_cur IS SELECT * FROM employee; Step 2: Once you have declared the cursor, you can open it: OPEN employee_cur; Step 3: And then you can fetch data from it row by row, usually inside a loop FETCH employee_cur INTO employee_rec; In this case, each record fetched from this cursor represents an entire record in the employee table. Step 4: finally, You can close the cursor: CLOSE employee_cur;

Using an Explicit Cursor
Declare the cursor CURSOR cursor_name IS select_query; Open the cursor OPEN cursor_name; Fetch the data rows LOOP FETCH cursor_name INTO variable_name(s); EXIT WHEN cursor_name%NOTFOUND; Close the cursor CLOSE cursor_name;

Explicit Cursor with %ROWTYPE

Cursor FOR Loop Automatically opens the cursor, fetches the records, then closes the cursor FOR variable_name(s) IN cursor_name LOOP processing commands END LOOP; Cursor variables cannot be used outside loop

Using Cursor FOR Loop

Handling Runtime Errors in PL/SQL Programs
Runtime errors cause exceptions Exception handlers exist to deal with different error situations Exceptions cause program control to fall to exception section where exception is handled

Predefined Exceptions

Undefined Exceptions Less common errors Do not have predefined names
Must declare your own name for the exception code in the declaration section DECLARE e_exception_name EXCEPTION; PRAGMA EXCEPTION_INIT(e_exception_name, -Oracle_error_code);

User-Defined Exceptions
Not a real Oracle error Use to enforce business rules

Summary PL/SQL is a programming language for working with an Oracle database Scalar, composite and reference variables can be used The IF/THEN/ELSE decision control structure allows branching logic Five loop constructs allow repeating code Cursors are returned from queries and can be explicitly iterated over Exception handling is performed in the exception section. User defined exceptions help to enforce business logic

Summary PL/SQL is a programming language for working with an Oracle database Scalar, composite and reference variables can be used The IF/THEN/ELSE decision control structure allows branching logic Five loop constructs allow repeating code

What can you do using PL/SQL
Something procedural instead of declarative. Stored procedure Stored function Trigger Cursor manipulation Exception control ….

Stored procedure Procedure header includes name(a parameter list)
Procedure, a function that does not return output through return value. Prototype, signature, and definition of the procedure, Procedure header includes name(a parameter list) A Parameter list includes a list of parameters A parameter list includes parameter name, in or out mode, and parameter data type. Name Procedure body Stored Precompiled ? modular

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