mySQL "comparing" two tables to find "missing" links

I have two tables A, B
A has a field Ptr which can be 0 or can point to an Index of B

I need to know which indices of B are not pointed to by A.
i.e., which ptrs are missing from A.

Again, note that A has extra fields where the ptr is 0 and that's okay.

I could make another field in B (and later erase it) to "tally" if A points to it, and then i have the missing indices, but i'm guessing there's a more elegant way to JOIN or compare to accomplish this.

thanks
willsherwoodAsked:
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Sudaraka WijesingheWeb Application ProgrammerCommented:
SELECT
	B.*
FROM B
LEFT JOIN A ON A.Ptr = B.Index
HAVING A.Ptr IS NULL

Open in new window

or
SELECT
	*
FROM B
WHERE Index NOT IN(SELECT Ptr FROM A)

Open in new window

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willsherwoodAuthor Commented:
many thanks, excellent!
0
Sudaraka WijesingheWeb Application ProgrammerCommented:
Glad to help. Thanks for the points.
0
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