标准偏差为什么除以n-1

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select * from tbl_Articles WHERE ArticleID=678380 LIMIT 0,1
select * from tbl_Articles_data WHERE ArticleID=678380 LIMIT 0,1
select * from tbl_Articles_sphinx where id=678380 LIMIT 0,1
SELECT ArticleID,Title FROM tbl_Articles WHERE ArticleID IN(2528295,3361428,1139773,1986962,752257,3191972,2819245,2047140,2541424,3468842,435464,603672,1144999,3491931,636812,1005911,2707579,3445628,2971808,3474493,2942247,3511605,1028838,723661,2697764,904708,983748,2613529,3181252,1033845) ORDER BY field (ArticleID,2528295,3361428,1139773,1986962,752257,3191972,2819245,2047140,2541424,3468842,435464,603672,1144999,3491931,636812,1005911,2707579,3445628,2971808,3474493,2942247,3511605,1028838,723661,2697764,904708,983748,2613529,3181252,1033845)