PL/SQL (Procedural Language/Structured Query Language) is Oracle Corporation's proprietary procedural extension to the SQL database language, used in the Oracle database. 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 1980s 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.
PL/SQL made its first appearance in Oracle Forms v3. A few years later, it was included in the Oracle Database server v7 (as database procedures, functions, packages, triggers and anonymous blocks) followed by Oracle Reports v2.
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:
Oracle Corporation customarily adds more packages and/or extends package functionality with each successive release of Oracle Database.
PL/SQL programs consist of procedures, functions, and anonymous blocks. Each of these is made up of the basic PL/SQL unit which is the block. Blocks take the general form:
Note that blocks can be nested within blocks.
DECLARE section specifies the datatypes of variables, constants, collections, and user-defined types.
The block between
END specifies executable procedural code.
Exceptions, errors which arise during the execution of the code, have one of two types:
Programmers have to raise user-defined exceptions explicitly. They can do this by using the RAISE command, with the syntax:
RAISEOracle Corporation has pre-defined several exceptions like
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
The DECLARE section defines and (optionally) initialises variables. If not initialised specifically they default to NULL.
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.
To Be Decided
Anonymous PL/SQL blocks can be embedded in an Oracle Precompiler or OCI program. At run time, the program, lacking a local PL/SQL engine, sends these blocks to the Oracle server, where they are compiled and executed. Likewise, interactive tools such as SQL*Plus and Enterprise Manager, lacking a local PL/SQL engine, must send anonymous blocks to Oracle.
To Be Decided
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:
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:
Oracle provides a number of data types that can store 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.
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 convert the dates to strings one uses the function
TO_CHAR (date_string, format_string).
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);
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';"
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.
The CASE statement simplifies some large IF-THEN-ELSE structures.
CASE statement can be used with predefined selector:
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.
WHILE condition LOOP
IN [REVERSE] .. LOOP
REVERSE keyword implements looping in reverse order.
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).
The concept of the person_code within the FOR-loop gets expressed with dot-notation ("."):
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's 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 "Advanced Data Type" (ADT), 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 Advanced Data Type are written in PL/SQL. The resulting Advanced 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 does not resemble Transact-SQL, despite superficial similarities due to the use of both as embedded database languages. 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.