Plsql 简明教程

PL/SQL - Strings

PL/SQL 中的字符串实际上是一系列字符，可选择指定大小。这些字符可以是数字、字母、空格、特殊字符或它们的组合。PL/SQL 提供了三种字符串——

The string in PL/SQL is actually a sequence of characters with an optional size specification. The characters could be numeric, letters, blank, special characters or a combination of all. PL/SQL offers three kinds of strings −

  1. Fixed-length strings − In such strings, programmers specify the length while declaring the string. The string is right-padded with spaces to the length so specified.

  2. Variable-length strings − In such strings, a maximum length up to 32,767, for the string is specified and no padding takes place.

  3. Character large objects (CLOBs) − These are variable-length strings that can be up to 128 terabytes.

PL/SQL 字符串可以是变量或文本。字符串文本用引号括起来。例如，

PL/SQL strings could be either variables or literals. A string literal is enclosed within quotation marks. For example,

'This is a string literal.' Or 'hello world'

要在一串文字中包含一个单引号，您需要将两个单引号放在一起。例如，

To include a single quote inside a string literal, you need to type two single quotes next to one another. For example,

'this isn''t what it looks like'

Declaring String Variables

Oracle 数据库提供了多种字符串数据类型，例如 CHAR、NCHAR、VARCHAR2、NVARCHAR2、CLOB 和 NCLOB。前缀为 'N' 的数据类型是 'national character set' 数据类型，用于存储 Unicode 字符数据。

Oracle database provides numerous string datatypes, such as CHAR, NCHAR, VARCHAR2, NVARCHAR2, CLOB, and NCLOB. The datatypes prefixed with an 'N' are 'national character set' datatypes, that store Unicode character data.

如果您需要声明一个可变长度字符串，您必须提供该字符串的最大长度。例如，VARCHAR2 数据类型。以下示例说明如何声明并使用一些字符串变量——

If you need to declare a variable-length string, you must provide the maximum length of that string. For example, the VARCHAR2 data type. The following example illustrates declaring and using some string variables −

DECLARE
   name varchar2(20);
   company varchar2(30);
   introduction clob;
   choice char(1);
BEGIN
   name := 'John Smith';
   company := 'Infotech';
   introduction := ' Hello! I''m John Smith from Infotech.';
   choice := 'y';
   IF choice = 'y' THEN
      dbms_output.put_line(name);
      dbms_output.put_line(company);
      dbms_output.put_line(introduction);
   END IF;
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 −

John Smith
Infotech
Hello! I'm John Smith from Infotech.

PL/SQL procedure successfully completed

若要声明定长字符串，请使用 CHAR 数据类型。在此处，您不必为定长变量指定最大长度。如果您省略长度约束，Oracle Database 会自动使用所需的最小长度。下列两个声明是相同的 −

To declare a fixed-length string, use the CHAR datatype. Here you do not have to specify a maximum length for a fixed-length variable. If you leave off the length constraint, Oracle Database automatically uses a maximum length required. The following two declarations are identical −

red_flag CHAR(1) := 'Y';
 red_flag CHAR   := 'Y';

PL/SQL String Functions and Operators

PL/SQL 提供连接符运算符 (||) ，用于连接两个字符串。下表提供了 PL/SQL 提供的字符串函数 −

PL/SQL offers the concatenation operator (||) for joining two strings. The following table provides the string functions provided by PL/SQL −

S.No

Function & Purpose

1

ASCII(x); Returns the ASCII value of the character x.

2

CHR(x); Returns the character with the ASCII value of x.

3

CONCAT(x, y); Concatenates the strings x and y and returns the appended string.

4

INITCAP(x); Converts the initial letter of each word in x to uppercase and returns that string.

5

INSTR(x, find_string [, start] [, occurrence]); Searches for find_string in x and returns the position at which it occurs.

6

INSTRB(x); Returns the location of a string within another string, but returns the value in bytes.

7

LENGTH(x); Returns the number of characters in x.

8

LENGTHB(x); Returns the length of a character string in bytes for single byte character set.

9

LOWER(x); Converts the letters in x to lowercase and returns that string.

10

LPAD(x, width [, pad_string]) ; Pads x with spaces to the left, to bring the total length of the string up to width characters.

11

LTRIM(x [, trim_string]); Trims characters from the left of x.

12

NANVL(x, value); Returns value if x matches the NaN special value (not a number), otherwise x is returned.

13

NLS_INITCAP(x); Same as the INITCAP function except that it can use a different sort method as specified by NLSSORT.

14

NLS_LOWER(x) ; Same as the LOWER function except that it can use a different sort method as specified by NLSSORT.

15

NLS_UPPER(x); Same as the UPPER function except that it can use a different sort method as specified by NLSSORT.

16

NLSSORT(x); Changes the method of sorting the characters. Must be specified before any NLS function; otherwise, the default sort will be used.

17

NVL(x, value); Returns value if x is null; otherwise, x is returned.

18

NVL2(x, value1, value2); Returns value1 if x is not null; if x is null, value2 is returned.

19

REPLACE(x, search_string, replace_string); Searches x for search_string and replaces it with replace_string.

20

RPAD(x, width [, pad_string]); Pads x to the right.

21

RTRIM(x [, trim_string]); Trims x from the right.

22

SOUNDEX(x) ; Returns a string containing the phonetic representation of x.

23

SUBSTR(x, start [, length]); Returns a substring of x that begins at the position specified by start. An optional length for the substring may be supplied.

24

SUBSTRB(x); Same as SUBSTR except that the parameters are expressed in bytes instead of characters for the single-byte character systems.

25

TRIM([trim_char FROM) x); Trims characters from the left and right of x.

26

UPPER(x); Converts the letters in x to uppercase and returns that string.

现在让我们通过几个示例来理解这个概念 -

Let us now work out on a few examples to understand the concept −

Example 1

DECLARE
   greetings varchar2(11) := 'hello world';
BEGIN
   dbms_output.put_line(UPPER(greetings));

   dbms_output.put_line(LOWER(greetings));

   dbms_output.put_line(INITCAP(greetings));

   /* retrieve the first character in the string */
   dbms_output.put_line ( SUBSTR (greetings, 1, 1));

   /* retrieve the last character in the string */
   dbms_output.put_line ( SUBSTR (greetings, -1, 1));

   /* retrieve five characters,
      starting from the seventh position. */
   dbms_output.put_line ( SUBSTR (greetings, 7, 5));

   /* retrieve the remainder of the string,
      starting from the second position. */
   dbms_output.put_line ( SUBSTR (greetings, 2));

   /* find the location of the first "e" */
   dbms_output.put_line ( INSTR (greetings, 'e'));
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
hello world
Hello World
h
d
World
ello World
2

PL/SQL procedure successfully completed.

Example 2

DECLARE
   greetings varchar2(30) := '......Hello World.....';
BEGIN
   dbms_output.put_line(RTRIM(greetings,'.'));
   dbms_output.put_line(LTRIM(greetings, '.'));
   dbms_output.put_line(TRIM( '.' from greetings));
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
Hello World.....
Hello World

PL/SQL procedure successfully completed.