SELECT  *
FROM    mytable
WHERE   descendant = @descendant
        AND hops < 
        (
        SELECT  hops
        FROM    mytable
        WHERE   descendant = @descendant
                AND ancestor = @ancestor
        )

这将自动处理@ancestor不是真正的@descendant祖先的情况.

This will automatically take care of cases when @ancestor is not really a @descendant's ancestor.

在(descendant, hops)上创建索引以使其快速运行.

Create an index on (descendant, hops) for this to work fast.