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📜  PL / SQL-字符串

📅  最后修改于: 2020-11-26 05:53:51             🧑  作者: Mango

PL / SQL中的字符串实际上是具有可选大小规格的字符序列。字符可以是数字，字母，空格，特殊字符或所有字符的组合。 PL / SQL提供三种字符串-

  • 固定长度的字符串-在此类字符串，程序员在声明字符串指定长度。该字符串用空格右填充，直到指定的长度为止。

  • 可变长度字符串-在此类字符串，指定了最大长度为32,767的字符串，并且不进行填充。

  • 字符大对象(CLOB) -这些是长度可变的字符串，最大可为128 TB。

PL / SQL字符串可以是变量，也可以是字面量。字符串字面量引号引起来。例如，

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

要将单引号包含在字符串字面量，您需要彼此相邻地键入两个单引号。例如，

'this isn''t what it looks like'

声明字符串变量

Oracle数据库提供了许多字符串数据类型，例如CHAR，NCHAR，VARCHAR2，NVARCHAR2，CLOB和NCLOB。带有“ N”前缀的数据类型是“国家字符集”数据类型，用于存储Unicode字符数据。

如果需要声明可变长度的字符串，则必须提供该字符串的最大长度。例如，VARCHAR2数据类型。以下示例说明了声明和使用一些字符串变量-

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，将产生以下结果-

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

PL/SQL procedure successfully completed

要声明固定长度的字符串，请使用CHAR数据类型。在这里，您不必为固定长度的变量指定最大长度。如果不考虑长度限制，Oracle数据库将自动使用所需的最大长度。以下两个声明是相同的-

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

PL / SQL字符串函数和运算符

PL / SQL提供了用于连接两个字符串的串联运算符(||) 。下表提供了PL / SQL提供的字符串函数-

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.

现在让我们通过一些例子来理解这个概念-

例子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，将产生以下结果-

HELLO WORLD 
hello world 
Hello World 
h 
d 
World 
ello World 
2  

PL/SQL procedure successfully completed.

例子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，将产生以下结果-

......Hello World  
Hello World..... 
Hello World  

PL/SQL procedure successfully completed.