PL/SQL - Strings

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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:

  • 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.

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

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

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'

To include a single quote inside a string literal, you need to type two single quotes next to one another, like:

'this isn''t what it looks like'

Declaring String Variables

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

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;
/

When the above code is executed at SQL prompt, it produces the following result:

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

PL/SQL procedure successfully completed

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. So following two declarations below are identical:

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

PL/SQL String Functions and Operators

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

S.N.Function & Purpose
1ASCII(x);
Returns the ASCII value of the character x.
2CHR(x);
Returns the character with the ASCII value of x.
3CONCAT(x, y);
Concatenates the strings x and y and return the appended string.
4INITCAP(x);
Converts the initial letter of each word in x to uppercase and returns that string.
5INSTR(x, find_string [, start] [, occurrence]);
Searches for find_string in x and returns the position at which it occurs.
6INSTRB(x);
Returns the location of a string within another string, but returns the value in bytes.
7LENGTH(x);
Returns the number of characters in x.
8LENGTHB(x);
Returns the length of a character string in bytes for single byte character set.
9LOWER(x);
Converts the letters in x to lowercase and returns that string.
10LPAD(x, width [, pad_string]) ;
Pads x with spaces to 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.
12NANVL(x, value);
Returns value if x matches the NaN special value (not a number), otherwise x is returned.
13NLS_INITCAP(x);
Same as the INITCAP function except that it can use a different sort method as specified by NLSSORT.
14NLS_LOWER(x) ;
Same as the LOWER function except that it can use a different sort method as specified by NLSSORT.
15NLS_UPPER(x);
Same as the UPPER function except that it can use a different sort method as specified by NLSSORT.
16NLSSORT(x);
Changes the method of sorting the characters. Must be specified before any NLS function; otherwise, the default sort will be used.
17NVL(x, value);
Returns value if x is null; otherwise, x is returned.
18NVL2(x, value1, value2);
Returns value1 if x is not null; if x is null, value2 is returned.
19REPLACE(x, search_string, replace_string);
Searches x for search_string and replaces it with replace_string.
20RPAD(x, width [, pad_string]);
Pads x to the right.
21RTRIM(x [, trim_string]);
Trims x from the right.
22SOUNDEX(x) ;
Returns a string containing the phonetic representation of x.
23SUBSTR(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.
24SUBSTRB(x);
Same as SUBSTR except the parameters are expressed in bytes instead of characters for the single-byte character systems.
25TRIM([trim_char FROM) x);
Trims characters from the left and right of x.
26UPPER(x);
Converts the letters in x to uppercase and returns that string.

The following examples illustrate some of the above-mentioned functions and their use:

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;
/

When the above code is executed at 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;
/

When the above code is executed at SQL prompt, it produces the following result:

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

PL/SQL procedure successfully completed.


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