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SQL: comments

The SQL standard defines two kinds of comments:

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 a 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.