mysql-问一下各位大神sql的问题
mysqljava报表
我要做一张报表，统计最近三十天内 进货、销售、库存每天的数量，已知有四张表
A表有前五个字段，
B表进货表有第一列的数据 可以count取出，有一个字段x存的时间
C表销售表有第二列的数据 可以count取出，有一个字段x存的时间
D表库存表有第三列的数据 可以count取出，有一个字段x存的时间
但是有一个问题，不是每天都有进货、销售、库存数据
问一下大神怎么用mysql查询符合这样报表的数据？
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