PL/SQL is a block-structured language, meaning that PL/SQL programs are divided and written in logical blocks of code. Each block consists of three sub-parts:
| S.N. | Sections & Description |
|---|---|
| 1 |
Declarations
This section starts with the keyword DECLARE. It is an optional section and defines all variables, cursors, subprograms, and other elements to be used in the program.
|
| 2 |
Executable Commands
This section is enclosed between the keywords BEGIN and END and it is a mandatory section. It consists of the executable PL/SQL statements of the program. It should have at least one executable line of code, which may be just a NULL command to indicate that nothing should be executed.
|
| 3 |
Exception Handling
This section starts with the keyword EXCEPTION. This section is again optional and contains exception(s) that handle errors in the program.
|
Every PL/SQL statement ends with a semicolon (;). PL/SQL blocks can be nested within other PL/SQL blocks using BEGIN and END. Here is the basic structure of a PL/SQL block:
DECLARE <declarations section> BEGIN <executable command(s)> EXCEPTION <exception handling> END;
The 'Hello World' Example:
DECLARE message varchar2(20):= 'Hello, World!'; BEGIN dbms_output.put_line(message); END; /
The end; line signals the end of the PL/SQL block. To run the code from SQL command line, you may need to type / at the beginning of the first blank line after the last line of the code. When the above code is executed at SQL prompt, it produces the following result:
Hello World PL/SQL procedure successfully completed.
The PL/SQL Identifiers
PL/SQL identifiers are constants, variables, exceptions, procedures, cursors, and reserved words. The identifiers consist of a letter optionally followed by more letters, numerals, dollar signs, underscores, and number signs and should not exceed 30 characters.
By default, identifiers are not case-sensitive. So you can use integer or INTEGER to represent a numeric value. You cannot use a reserved keyword as an identifier.
The PL/SQL Delimiters
A delimiter is a symbol with a special meaning. Following is the list of delimiters in PL/SQL:
| Delimiter | Description |
|---|---|
| +, -, *, / | Addition, subtraction/negation, multiplication, division |
| % | Attribute indicator |
| ' | Character string delimiter |
| . | Component selector |
| (,) | Expression or list delimiter |
| : | Host variable indicator |
| , | Item separator |
| " | Quoted identifier delimiter |
| = | Relational operator |
| @ | Remote access indicator |
| ; | Statement terminator |
| := | Assignment operator |
| => | Association operator |
| || | Concatenation operator |
| ** | Exponentiation operator |
| <<, >> | Label delimiter (begin and end) |
| /*, */ | Multi-line comment delimiter (begin and end) |
| -- | Single-line comment indicator |
| .. | Range operator |
| <, >, <=, >= | Relational operators |
| <>, '=, ~=, ^= | Different versions of NOT EQUAL |
The PL/SQL Comments
Program comments are explanatory statements that you can include in the PL/SQL code that you write and helps anyone reading its source code. All programming languages allow for some form of comments.
The PL/SQL supports single-line and multi-line comments. All characters available inside any comment are ignored by PL/SQL compiler. The PL/SQL single-line comments start with the delimiter -- (double hyphen) and multi-line comments are enclosed by /* and */.
DECLARE -- variable declaration message varchar2(20):= 'Hello, World!'; BEGIN /* * PL/SQL executable statement(s) */ dbms_output.put_line(message); END; /
When the above code is executed at SQL prompt, it produces the following result:
Hello World PL/SQL procedure successfully completed.
PL/SQL Program Units
A PL/SQL unit is any one of the following:
- PL/SQL block
- Function
- Package
- Package body
- Procedure
- Trigger
- Type
- Type body
Each of these units will be discussed in the forthcoming chapters.
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