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PL/SQL (Procedural Language/Structured Query Language) is Oracle Corporation's procedural language extension for SQL and the Oracle relational database. PL/SQL is available in Oracle Database (since version 7), TimesTen in-memory database (since version 11.2.1), and IBM DB2 (since version 9.7). Oracle Corporation usually extends PL/SQL functionality with each successive release of the Oracle Database.
PL/SQL includes procedural language elements such as conditions and loops. You can declare constants and variables, procedures and functions, types and variables of those types, and triggers. You can handle exceptions (runtime errors). Arrays are supported involving the use of PL/SQL collections. Implementations from version 8 of Oracle Database onwards have included features associated with object-orientation. You can create PL/SQL units—procedures, functions, packages, types, and triggers —that are stored in the database for reuse by applications that use any of the Oracle Database programmatic interfaces.
PL/SQL functions analogously to the embedded procedural languages associated with other relational databases. Sybase ASE and Microsoft SQL Server have Transact-SQL, PostgreSQL has PL/pgSQL (which tries to emulate PL/SQL to an extent), and IBM DB2 includes SQL Procedural Language, which conforms to the ISO SQL’s SQL/PSM standard.
The designers of PL/SQL modelled its syntax on that of Ada. Both Ada and PL/SQL have Pascal as a common ancestor, and so PL/SQL also resembles Pascal in numerous aspects. The structure of a PL/SQL package closely resembles the basic Pascal program structure or a Borland Delphi unit. Programmers can define global data-types, constants and static variables, public and private, in a PL/SQL package.
PL/SQL also allows for the definition of classes and instantiating these as objects in PL/SQL code. This resembles usages in object-oriented programming languages like Object Pascal, C++ and Java. PL/SQL refers to a class as an "Abstract Data Type" (ADT) or "User Defined Type" (UDT), and defines it as an Oracle SQL data-type as opposed to a PL/SQL user-defined type, allowing its use in both the Oracle SQL Engine and the Oracle PL/SQL engine. The constructor and methods of an Abstract Data Type are written in PL/SQL. The resulting Abstract Data Type can operate as an object class in PL/SQL. Such objects can also persist as column values in Oracle database tables.
PL/SQL is fundamentally distinct from Transact-SQL, despite superficial similarities. Porting code from one to the other usually involves non-trivial work, not only due to the differences in the feature sets of the two languages, but also due to the very significant differences in the way Oracle and SQL Server deal with concurrency and locking.
A PL/SQL program unit is one of the following: PL/SQL Anonymous Block, Procedure, Function, Package Specification, Package Body, Trigger, Type Specification, Type Body, Library. Program units are the PL/SQL source code that is compiled, developed and ultimately executed on the database.
The basic unit of a PL/SQL source program is the block, which groups related declarations and statements. A PL/SQL block is defined by the keywords DECLARE, BEGIN, EXCEPTION, and END. These keywords divide the block into a declarative part, an executable part, and an exception-handling part. The declaration section is optional and may be used to define and initialize constants and variables. If a variable is not initialized then it defaults to NULL value. The optional exception-handling part is used to handle run time errors. Only the executable part is required. A block can have a label.
<<label>> -- this is optional DECLARE -- this section is optional number1 NUMBER(2); number2 number1%TYPE := 17; -- value default text1 varchar4(12) := 'Hello world'; text2 DATE := SYSDATE; -- current date and time BEGIN -- this section is mandatory, must contain at least one executable statement SELECT street_number INTO number1 FROM address WHERE name = 'INU'; EXCEPTION -- this section is optional WHEN OTHERS THEN DBMS_OUTPUT.PUT_LINE('Error Code is ' || TO_CHAR(SQLCODE ) ); DBMS_OUTPUT.PUT_LINE('Error Message is ' || SQLERRM ); END;
The symbol := functions as an assignment operator to store a value in a variable.
Blocks can be nested: Because a block is an executable statement, it can appear in another block wherever an executable statement is allowed. You can submit a block to an interactive tool (such as SQL*Plus) or embed it in an Oracle Precompiler or OCI program. The interactive tool or program runs the block one time. The block is not stored in the database, and for that reason, it is called an anonymous block (even if it has a label).
The purpose of a PL/SQL function is generally to compute and return a single value. This returned value may be a single scalar value (such as a number, date or character string) or a single collection (such as a nested table or varray). User-defined functions supplement the built-in functions provided by Oracle Corporation.
The PL/SQL function has the form:
CREATE OR REPLACE FUNCTION <function_name> [(input/output variable declarations)] RETURN return_type [AUTHID <CURRENT_USER | DEFINER>] <IS|AS> -- heading part amount NUMBER;-- declaration block] BEGIN -- exectutable part <PL/SQL block WITH RETURN statement> RETURN <return_value>; [EXCEPTION none] RETURN <return_value>; END;
A pipelined table functions return collections and take the form:
CREATE OR REPLACE FUNCTION <function_name> [(input/output variable declarations)] RETURN return_type [AUTHID <CURRENT_USER | DEFINER>] [<AGGREGATE | PIPELINED>] <IS|USING> [declaration block] BEGIN <PL/SQL block WITH RETURN statement> PIPE ROW <RETURN type>; RETURN; [EXCEPTION EXCEPTION block] PIPE ROW <RETURN type>; RETURN; END;
A function should only use the default IN type of parameter. The only out value from the function should be the value it returns.
Procedures are similar to Functions, in that they are named program units that can be invoked repeatedly. The primary difference is that functions can be used in a SQL statement whereas procedures cannot. Another difference is that the procedure can return multiple values whereas a function should only return a single value.
The procedure begins with a mandatory heading part to hold the procedure name and optionally the procedure parameter list. Next are the declarative, executable and exception-handling parts, as in the PL/SQL Anonymous Block. Here is an example of a simple procedure.
CREATE PROCEDURE create_email_address ( -- Procedure heading part begins name1 VARCHAR2, name2 VARCHAR2, company VARCHAR2 ) -- Procedure heading part ends AS -- Declarative part begins (optional) error_message VARCHAR2(30) := 'Email address is too long.'; BEGIN -- Executable part begins (mandatory) email := name1 || '.' || name2 || '@' || company; EXCEPTION -- Exception-handling part begins (optional) WHEN VALUE_ERROR THEN DBMS_OUTPUT.PUT_LINE(error_message); END create_email_address;
The example above shows a Standalone Procedure - this type of procedure is created and stored in a database schema using the CREATE PROCEDURE statement. A procedure may also be created in a PL/SQL package - this is called a Package Procedure. A procedure created in a PL/SQL anonymous block is called a Nested Procedure. The Standalone or Package procedures are stored in the database and so are referred to as Stored Procedures.
There are three types of parameter: IN, OUT and IN OUT.
Packages are groups of conceptually linked functions, procedures, variables, PL/SQL table and record TYPE statements, constants, cursors etc. The use of packages promotes re-use of code. Packages are composed of the package specification and an optional package body. The specification is the interface to the application; it declares the types, variables, constants, exceptions, cursors, and subprograms available. The body fully defines cursors and subprograms, and so implements the spec. Two advantages of packages include:
A trigger is like a stored procedure that Oracle Database invokes automatically whenever a specified event occurs. It is a named PL/SQL unit that is stored in the database and can be invoked repeatedly. Unlike a stored procedure, you can enable and disable a trigger, but you cannot explicitly invoke it. While a trigger is enabled, the database automatically invokes it—that is, the trigger fires—whenever its triggering event occurs. While a trigger is disabled, it does not fire.
You create a trigger with the CREATE TRIGGER statement. You specify the triggering event in terms of triggering statements and the item on which they act. The trigger is said to be created on or defined on the item, which is either a table, a view, a schema, or the database. You also specify the timing point, which determines whether the trigger fires before or after the triggering statement runs and whether it fires for each row that the triggering statement affects.
If the trigger is created on a table or view, then the triggering event is composed of DML statements, and the trigger is called a DML trigger. If the trigger is created on a schema or the database, then the triggering event is composed of either DDL or database operation statements, and the trigger is called a system trigger.
An INSTEAD OF trigger is either: A DML trigger created on a view or a system trigger defined on a CREATE statement. The database fires the INSTEAD OF trigger instead of running the triggering statement.
Triggers can be written for the following purposes:
Generating some derived column values automatically
Enforcing referential integrity
Event logging and storing information on table access
Synchronous replication of tables
Imposing security authorizations
Preventing invalid transactions
The major datatypes in PL/SQL include NUMBER, INTEGER, CHAR, VARCHAR2, DATE and TIMESTAMP.
variable_name NUMBER(P[,S]) := 0;
To define a numeric variable, the programmer appends the variable type NUMBER to the name definition. To specify the (optional) precision (P) and the (optional) scale (S), one can further append these in round brackets, separated by a comma. ("Precision" in this context refers to the number of digits which the variable can hold, "scale" refers to the number of digits which can follow the decimal point.)
A selection of other datatypes for numeric variables would include: binary_float, binary_double, dec, decimal, double precision, float, integer, int, numeric, real, smallint, binary_integer.
variable_name VARCHAR2(10) := 'Text';
To define a character variable, the programmer normally appends the variable type VARCHAR2 to the name definition. There follows in brackets the maximum number of characters which the variable can store.
Other datatypes for character variables include: varchar, char, long, raw, long raw, nchar, nchar2, clob, blob, bfile
variable_name DATE := TO_DATE('01-01-2005 14:20:23','DD-MM-YYYY hh24:mi:ss');
Date variables can contain date and time. The time may be left out, but there is no way to define a variable that only contains the time. There is no DATETIME type. And there is no TIME type. But there is a TIMESTAMP type that can contain fine grained timestamp up to millisecond or nanosecond. Oracle Datatypes
TO_DATE function can be used to convert strings to date values. The function converts the first quoted string into a date, using as a definition the second quoted string, for example:
TO_DATE ('31-Dec-2004','dd-mon-yyyy', 'NLS_DATE_LANGUAGE = American')
To convert the dates to strings one uses the function
TO_CHAR (date_string, format_string).
PL/SQL also supports the use of ANSI date and interval literals. The following clause gives an 18-month range:
WHERE dateField BETWEEN DATE '2004-12-30' - INTERVAL '1-6' YEAR TO MONTH AND DATE '2004-12-30'
Exceptions, errors which arise during the execution of the code, have one of two types:
User-defined exceptions are always raised explicitly by the programmers, using the
RAISE_APPLICATION_ERROR commands, in any situation where they have determined that it is impossible for normal execution to continue. The
RAISE command has the syntax:
RAISE <EXCEPTION name>;
Oracle Corporation has predefined several exceptions like
TOO_MANY_ROWS, etc. Each exception has an SQL Error Number and SQL Error Message associated with it. Programmers can access these by using the
This syntax defines a variable of the type of the referenced column on the referenced tables.
Programmers specify user-defined datatypes with the syntax:
type data_type is record (field_1 type_1 :=xyz, field_2 type_2 :=xyz, ..., field_n type_n :=xyz);
DECLARE TYPE t_address IS RECORD ( name address.name%TYPE, street address.street%TYPE, street_number address.street_number%TYPE, postcode address.postcode%TYPE); v_address t_address; BEGIN SELECT name, street, street_number, postcode INTO v_address FROM address WHERE ROWNUM = 1; END;
This sample program defines its own datatype, called t_address, which contains the fields name, street, street_number and postcode.
So according to the example, we are able to copy the data from the database to the fields in the program.
Using this datatype the programmer has defined a variable called v_address and loaded it with data from the ADDRESS table.
Programmers can address individual attributes in such a structure by means of the dot-notation, thus: "v_address.street := 'High Street';"
The following code segment shows the IF-THEN-ELSIF construct. The ELSIF and ELSE parts are optional so it is possible to create simpler IF-THEN or, IF-THEN-ELSE constructs.
IF x = 1 THEN sequence_of_statements_1; ELSIF x = 2 THEN sequence_of_statements_2; ELSIF x = 3 THEN sequence_of_statements_3; ELSIF x = 4 THEN sequence_of_statements_4; ELSIF x = 5 THEN sequence_of_statements_5; ELSE sequence_of_statements_N; END IF;
The CASE statement simplifies some large IF-THEN-ELSE structures.
CASE WHEN x = 1 THEN sequence_of_statements_1; WHEN x = 2 THEN sequence_of_statements_2; WHEN x = 3 THEN sequence_of_statements_3; WHEN x = 4 THEN sequence_of_statements_4; WHEN x = 5 THEN sequence_of_statements_5; ELSE sequence_of_statements_N; END CASE;
CASE statement can be used with predefined selector:
CASE x WHEN 1 THEN sequence_of_statements_1; WHEN 2 THEN sequence_of_statements_2; WHEN 3 THEN sequence_of_statements_3; WHEN 4 THEN sequence_of_statements_4; WHEN 5 THEN sequence_of_statements_5; ELSE sequence_of_statements_N; END CASE;
PL/SQL refers to arrays as "collections". The language offers three types of collections:
Programmers must specify an upper limit for varrays, but need not for index-by tables or for nested tables. The language includes several collection methods used to manipulate collection elements: for example FIRST, LAST, NEXT, PRIOR, EXTEND, TRIM, DELETE, etc. Index-by tables can be used to simulate associative arrays, as in this example of a memo function for Ackermann's function in PL/SQL.
With index-by tables, the array can be indexed by numbers or strings. It parallels a Java map, which comprises key-value pairs. There is only one dimension and it is unbounded.
With nested tables the programmer needs to understand what is nested. Here, a new type is created that may be composed of a number of components. That type can then be used to make a column in a table, and nested within that column will be those components.
With Varrays you need to understand that the word "variable" in the phrase "variable-size arrays" doesn't apply to the size of the array in the way you might think that it would. The size the array is declared with is in fact fixed. The number of elements in the array is variable up to the declared size. Arguably then, variable-sized arrays aren't that variable in size.
<<parent_loop>> LOOP statements <<child_loop>> LOOP statements EXIT parent_loop WHEN <condition>; -- Terminates both loops EXIT WHEN <condition>; -- Returns control to parent_loop END LOOP child_loop; IF <condition> THEN continue; -- continue to next iteration END IF; EXIT WHEN <condition>; END LOOP parent_loop;
DECLARE var NUMBER; BEGIN /*N.B. for loop variables in pl/sql are new declarations, with scope only inside the loop */ FOR var IN 0 .. 10 LOOP DBMS_OUTPUT.put_line(var); END LOOP; IF (var IS NULL) THEN DBMS_OUTPUT.put_line('var is null'); ELSE DBMS_OUTPUT.put_line('var is not null'); END IF; END;
This is the output
0 1 2 3 4 5 6 7 8 9 10 var is null
FOR RecordIndex IN (SELECT person_code FROM people_table) LOOP DBMS_OUTPUT.PUT_LINE(RecordIndex.person_code); END LOOP;
Cursor-for loops automatically open a cursor, read in their data and close the cursor again.
As an alternative, the PL/SQL programmer can pre-define the cursor's SELECT-statement in advance in order (for example) to allow re-use or to make the code more understandable (especially useful in the case of long or complex queries).
DECLARE CURSOR cursor_person IS SELECT person_code FROM people_table; BEGIN FOR RecordIndex IN cursor_person LOOP DBMS_OUTPUT.PUT_LINE(recordIndex.person_code); END LOOP; END;
The concept of the person_code within the FOR-loop gets expressed with dot-notation ("."):
While programmers can readily embed Data Manipulation Language (DML) statements directly into their PL/SQL code using straightforward SQL statements, Data Definition Language (DDL) requires more complex "Dynamic SQL" statements to be written in the PL/SQL code. However, DML statements underpin the majority of PL/SQL code in typical software applications.
In the case of PL/SQL dynamic SQL, early versions of the Oracle Database required the use of a complicated Oracle
DBMS_SQL package library. More recent versions have however introduced a simpler "Native Dynamic SQL", along with an associated
EXECUTE IMMEDIATE syntax.
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