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  • Possible duplicate of difference between primary key and unique key Commented Nov 23, 2016 at 14:00
  • e4c5 I'm not asking for difference, I'm asking why anyone should set both of these constraints for the same field. Maybe there's a reason behind that. Commented Nov 23, 2016 at 14:02
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    @e4c5, how do you think it's a duplicate of that linked answer? can you explain. Commented Nov 23, 2016 at 14:07
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    There's a very nice upside of doing this - knowing who did that tells you that it's a person who you don't want as your co-worker. It's blatantly stupid to do this, it makes no sense. It's like adding more water to a glass of water to make water wetter or adding white color to white color to make it whiter. The guy who did that had no clue what he was doing at that point of his IT life. Classic human. Commented Nov 23, 2016 at 16:05
  • haha you made my day @Mjh! Commented Nov 24, 2016 at 14:26