CONCAT WITH ||
The || is an operator which is used to simplify the work of the CONCAT function and thus is used to concatenate two or more strings together. The || operator is supported in the various versions of the Oracle/PLSQL, including, Oracle 12c, Oracle 11g, Oracle 10g, Oracle 9i and Oracle 8i.
Syntax:
string_1 || string_2 || string_n
 Parameters: 
String_1:  It is used to specify the first string to concatenate.
String_2:  It is used to specify the second string to concatenate.
String_n: It is used to specify the nth string to concatenate.
Example 1: Concatenation of two strings.
|  ‘HELLO’ || ‘ WORLD’ | 
Output:
‘HELLO WORLD’
 Explanation: 
The two strings are concatenated as one string.
 Example 2: Concatenation of strings with space. 
| SELECT ‘HELLO’ || ‘ ’ || ‘WORLD’ FROM dual; | 
Output:
‘HELLO WORLD’
 Explanation: 
All the three strings are concatenated as one string.
Example 3: Concatenation of strings with single quotes.
| SELECT ‘LET’‘S’ || ‘ BE’ || ' FRIENDS' FROM dual; | 
Output:
‘LET’S BE FRIENDS’
 Explanation: 
All the three strings are concatenated as one string.
Example 4: Concatenation of strings with single quotes (second method).
| SELECT ‘LET’ || “ ” || ‘S’ || ‘ BE’ || ' FRIENDS' FROM dual; | 
Output:
‘LET’S BE FRIENDS’
 Explanation: 
All the four strings are concatenated as one string.