Plsql 简明教程
PL/SQL - Basic Syntax
在本章中，我们将讨论 PL/SQL 的基本语法，这是一种 block-structured 语言；这意味着 PL/SQL 程序被划分为逻辑代码块并编写在这些逻辑代码块中。每个块包含三个子部分：−
In this chapter, we will discuss the Basic Syntax of PL/SQL which is a block-structured language; this means that the PL/SQL programs are divided and written in logical blocks of code. Each block consists of three sub-parts −
S.No |
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 optional section contains exception(s) that handle errors in the program. |
每个 PL/SQL 语句都以分号 (;) 结尾。PL/SQL 块可以使用 BEGIN 和 END 嵌套在其他 PL/SQL 块内。以下是 PL/SQL 块的基本结构：−
Every PL/SQL statement ends with a semicolon (;). PL/SQL blocks can be nested within other PL/SQL blocks using BEGIN and END. Following 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;
/end; 行表示 PL/SQL 块的结束。要在 SQL 命令行中运行代码，您可能需要在代码最后一行之后的第一个空白行的开头键入 /。在 SQL 提示符处执行以上代码时，它产生以下结果：−
The end; line signals the end of the PL/SQL block. To run the code from the 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 the SQL prompt, it produces the following result −
Hello World
PL/SQL procedure successfully completed.The PL/SQL Identifiers
PL/SQL 标识符是常量、变量、异常、过程、游标和保留字。标识符由一个字母（后面可以跟更多字母、数字、美元符号、下划线和数字符号）组成，并且不应超过 30 个字符。
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.
默认情况下， identifiers are not case-sensitive 。因此，你可以使用 integer 或者 INTEGER 来表示数值。你不能将保留关键字用作标识符。
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
分隔符是具有特殊含义的符号。以下是 PL/SQL 中的分隔符列表：
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
程序注释是可以包括在你所编写的 PL/SQL 代码中的解释性语句，并且有助于其他人阅读其源代码。所有编程语言都允许某种形式的注释。
Program comments are explanatory statements that can be included in the PL/SQL code that you write and helps anyone reading its source code. All programming languages allow some form of comments.
PL/SQL 支持单行和多行注释。PL/SQL 编译器会忽略出现在任何注释中的所有字符。PL/SQL 单行注释以分隔符 (双连字符) 开始，多行注释则由 /* 和 */ 括住。
The PL/SQL supports single-line and multi-line comments. All characters available inside any comment are ignored by the 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;
/当以上代码在 SQL 提示符下执行时，它会生成以下结果：
When the above code is executed at the SQL prompt, it produces the following result −
Hello World
PL/SQL procedure successfully completed.