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SQL: comments
The
SQL standard defines two kinds of comments:
- Simple comments (Feature E161) and
- Bracketed comments (Feature T351)
A
simple comment starts with two hyphens (
--) and extends to the next
new line or end of statement, whichever is first.
A Bracketed comment starts with a /* and extends to the next */.
In a comment that is introduced by a --, the /* is not recognized as a comment introducer.
Similarly, within /* … */ the -- does not comment out the */.
Apparrently, some SQL dialects allow to nest
/* … */ comments (among which there is
T-SQL).
The following snippet tries to demonstrate all combinations in a dialect that does not allow nesting comments.. T is SQL text, c is comment:
TTT TTTT TT -- ccc ccc
TTT /* cc ccc
cc cccc
-- */ ccc ccc
ccc */ TTT
/* ccc ccc
ccc
ccc -- */ TTT
-- /* ccc ccc
TTT TTTT
TTT -- */ ccc
With these rules, its possible to comment out or in a block of code while testing SQL statements with just two hyphens as is is shown below:
-- /*
This text is not a comment.
By just removing the two hyphens
at the beginning, it turns
into a comment.
-- */
Misc
The statement parser considers comments to be equivalent to
new lines.
I believe that the standard can be interpreted to mean that bracketed comments can be nested.
See also
In
MySQL, comments can also be introduced with a
#. Additionally, it seems that
-- needs to be followed by a space to be recognized as a comment.