13是合数吗? 什么是合数？

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select * from tbl_Articles WHERE ArticleID=81366 LIMIT 0,1
select * from tbl_Articles_data WHERE ArticleID=81366 LIMIT 0,1
select * from tbl_Articles_sphinx where id=81366 LIMIT 0,1
SELECT ArticleID,Title FROM tbl_Articles WHERE ArticleID IN(3226048,798966,3062733,3180383,1210722,443659,2457989,1519760,3205846,2791189,483402,3413245,2016161,751318,3481594,3088762,434352,2145726,1814368,2917267,363765,2040412,2262012,439400,462993,981998,2562832,2518421,483708,782441) ORDER BY field (ArticleID,3226048,798966,3062733,3180383,1210722,443659,2457989,1519760,3205846,2791189,483402,3413245,2016161,751318,3481594,3088762,434352,2145726,1814368,2917267,363765,2040412,2262012,439400,462993,981998,2562832,2518421,483708,782441)