SELECT count(item)
FROM online_transaction_enc
INNER JOIN online_transaction_enc ON supplier_id
INNER JOIN products ON supplier_id
...
WHERE WHERE (date BETWEEN '2010-01-30 14:15:55' AND '2010-09-29 10:15:55')
;

此解决方案使用内部联接。我相信这就是你要找的。这里有更多的文档http://www.mysqltutorial.org/mysql-inner-join.aspx

试试这个：

select count(a.item) from 
    (select date, id, item from online_transaction_enc
    union 
    select date, id, item from offline_transaction_enc)a
    inner join 
    (
    select  date,id,supplier_id,productcode from online_transaction
    union 
    select  date,id,supplier_id,productcode from offline_transaction)b
    on a.date=b.date and a.id=b.id
    inner join supplier on b.supplier_id=supplier.supplier_id
    inner join products on b.productcode=products.product_code
    where a.date between '20180601' And '20180730' AND Type='household' AND country='Malaysia'