PL/SQL (Procedural Language/Structured Query Language) is Oracle Corporation's proprietary procedural extension to the SQL database language. Some other SQL database management systems offer similar extensions to the SQL language. PL/SQL's syntax strongly resembles that of Ada, and just like Ada compilers of the 80-s the PL/SQL runtime system uses Diana as intermediate representation.
The key strength of PL/SQL is its tight integration with the Oracle database.
PL/SQL is one of three languages embedded in the Oracle Database, the other two being SQL and Java History
PL/SQL made its first appearance in Oracle Forms v3. A few years later, it has included in the Oracle Database server v7 (as database procedures, functions, packages, triggers and anonymous blocks) followed by Oracle Reports v2.
Functionality
PL/SQL supports variables, conditions, arrays, and exceptions. Implementations from version 8 of Oracle Database onwards have included features associated with object-orientation.
The underlying SQL functions as a declarative language. Standard SQL—unlike some functional programming languages—does not require implementations to convert tail calls to jumps. The open standard SQL does not readily provide "first row" and "rest of table" accessors, and it cannot easily perform some constructs such as loops. PL/SQL, however, as a Turing-complete procedural language which fills in these gaps, allows Oracle database developers to interface with the underlying relational database in an imperative manner. SQL statements can make explicit in-line calls to PL/SQL functions, or can cause PL/SQL triggers to fire upon pre-defined Data Manipulation Language (DML) events.
PL/SQL stored procedures (functions, procedures, packages, and triggers) which perform DML get compiled into an Oracle database: to this extent their SQL code can undergo syntax-checking. Programmers working in an Oracle database environment can construct PL/SQL blocks of such functionality to serve as procedures, functions; or they can write in-line segments of PL/SQL within SQL*Plus scripts.
While programmers can readily incorporate SQL DML statements into PL/SQL (as cursor definitions, for example, or using the SELECT ... INTO syntax), Data Definition Language (DDL) statements such as CREATE TABLE/DROP INDEX etc require the use of "Dynamic SQL". Earlier versions of Oracle Database required the use of a complex built-in DBMS_SQL package for Dynamic SQL where the system needed to explicitly parse and execute an SQL statement. Later versions have included an EXECUTE IMMEDIATE syntax called "Native Dynamic SQL" which considerably simplifies matters. Any use of DDL in an Oracle database will result in an implicit commit. Programmers can also use Dynamic SQL to execute DML where they do not know the exact content of the statement in advance.
PL/SQL offers several pre-defined packages for specific purposes. Such PL/SQL packages include:
DBMS_OUTPUT - for output operations to non-database destinations
DBMS_JOB - for running specific procedures/functions at a particular time (i.e. scheduling)
DBMS_XPLAN - for formatting "Explain Plan" output
DBMS_SESSION - provides access to SQL ALTER SESSION and SET ROLE statements, and other session information.
DBMS_METADATA - for extracting meta data from the data dictionary (such as DDL statements)
UTL_FILE - for reading and writing files on disk
UTL_HTTP - for making requests to web servers from the database
UTL_SMTP - for sending mail from the database (via an SMTP server)
Oracle Corporation customarily adds more packages and/or extends package functionality with each successive release of Oracle Database.
Basic code structure
PL/SQL programs consist of blocks. Blocks take the general form:
DECLARE
-- Declaration block (optional)
BEGIN
-- Program proper
EXCEPTION
-- Exception-handling (optional)
END
/* Sample comment spanning
multiple lines... */
The DECLARE section specifies the datatypes of variables, constants, collections, and user-defined types.
The block between BEGIN and END specifies executable procedural code.
Exceptions, errors which arise during the execution of the code, have one of two types:
pre-defined exceptions
user-defined exceptions.
Programmers have to raise user-defined exceptions explicitly. They can do this by using the RAISE command, with the syntax:
RAISE
Oracle Corporation has pre-defined several exceptions like NO_DATA_FOUND, TOO_MANY_ROWS, etc. Each exception has a SQL Error Number and SQL Error Message associated with it. Programmers can access these by using the SQLCODE and SQLERRM functions.
Variables
The DECLARE section defines and (optionally) initialises variables. If not initialised specifically they default to NULL.
For example:
DECLARE
number1 NUMBER(2);
number2 NUMBER(2) := 17;
text1 VARCHAR2(12) := 'Hello world';
text2 DATE := SYSDATE; -- current date and time
BEGIN
SELECT street_number
INTO number1
FROM address
WHERE name = 'Smith';
END;
The symbol := functions as an assignment operator to store a value in a variable.
The major datatypes in PL/SQL include NUMBER, INTEGER, CHAR, VARCHAR2, DATE, TIMESTAMP, TEXT etc.
Numeric variables
variable_name number(P[,S]) := value;
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
Character variables
variable_name varchar2(L) := '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
Date variables
variable_name date := '01-Jan-2005';
Oracle provided a number of data types that can stores dates (DATE, DATETIME, TIMESTAMP etc), however DATE is most commonly used.
Programmers define date variables by appending the datatype code "DATE" to a variable name. The "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('31-12-2004','dd-mm-yyyy')
or
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).
Datatypes for specific columns
Variable_name Table_name.Column_name%type;
This syntax defines a variable of the type of the referenced column on the referenced table.
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);
For example:
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.
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';"
Conditional Statements
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;
Array handling
PL/SQL refers to arrays as "collections". The language offers three types of collections:
Index-by tables (associative arrays)
Nested tables
Varrays (variable-size arrays)
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, DELET