Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Programming
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Databases
Mail Systems
openSolaris
Eclipse Documentation
Techotopia.com
Virtuatopia.com

How To Guides
Virtualization
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Windows
Problem Solutions
Privacy Policy

  




 

 

Databases - Practical PostgreSQL
Previous Page Home Next Page

Comments

Comments are blocks of text that, through special character sequences, can embed non-SQL text within SQL code. These can be used within blocks of code, because PostgreSQL removes the commented areas from the input stream and treats it as whitespace. There are two styles of comments available: single-line comments, and multiline comments.

Single-line comments are preceded by two dashes (- -) and may either be on a line by themselves, or they may follow valid SQL tokens. (The comments themselves are not considered tokens to PostgreSQL's parser, as any character data following the - - sequence, up to the end of the line, is treated as whitespace.) This is demonstrated in Example 3-11.

Example 3-11. Single-line comments

testdb=# SELECT 'Test' -- This can follow valid SQL tokens,
testdb-#               -- or be on a line of it own.
testdb-# AS example;
 example
---------
 Test
(1 row)

Multiline comments begin with a sequential slash-asterisk (/*) sequence, and terminate with a sequential asterisk-slash (*/) sequence. This style of commenting may already be familiar to C programmers, but there is one key difference between PostgreSQL's interpreter and the C language interpreter: PostgreSQL comments may be nested . Therefore, when you create a multiline comment within another multiline comment, the */ used to close the inner comment does not also close the outer comment. Example 3-12 provides a comment explanation.

Example 3-12. Multiline comments

testdb=# SELECT 'Multi' /* This comment extends across
testdb*#                 * numerous lines, and can be
testdb*#                 * /* nested safely */ */
testdb-# || '-test' AS example;
  example
------------
 Multi-test
(1 row)

Nesting comments can be useful if you have a file containing SQL syntax of which you wish to comment a large portion before sending to PostgreSQL for interpreting and execution. If you have already used multiline comments within that document and you wish to comment a large section which includes those comments, PostgreSQL is intelligent enough to recognize that a closing comment sequence (*/) closes only the most recently opened comment, not the entire commented region.

Note: The asterisk character by itself (without an adjacent slash character) has no special meaning within a comment. The extra asterisks in Example 3-12 on multiline comments are provided only for aesthetic purposes and readability.

Databases - Practical PostgreSQL
Previous Page Home Next Page

 
 
  Published under the terms of the Open Publication License Design by Interspire