SELECT 
   ForeignKey 
FROM 
   FK_TABLE f 
LEFT JOIN 
   PK_TABLE p ON f.ForeignKey = p.PrimaryKey 
WHERE 
   p.PrimaryKey = NULL


That should do it.

ForeignKey = the column you want to make into a foreign key
PK_TABLE = the table you want the foreign key to reference
PrimaryKey = the column ForeignKey will be a foreign key to.

Finding columns that do not match existing primary key
Here and there I read of people who set their lens to a small aperture while testing their sensor cleanness, supposedly to get the best image of dust speckles. However, the image of the on-sensor dust particles should not, to my understanding, be affected by the sharpness induced by the lens settings. Same is true for dust particle on the lens elements themselves. It makes me wonder - do these people misunderstand the theory of how the optical system work, or am I missing something?
