有个问题始终想不通
left join 后应该记录数不变，为什么记录变少了？

select count (*) from product 有 9037条记录


select count (*) from product left join v_inv on product.prod_no=v_inv.prod_no where datediff(day,v_inv.inv_date,getdate())<30 只有 1330条记录？怎样能查到 9037条记录而又可以 关联v_inv?


[解决办法]
select count(*) from
(select * from product left join v_inv on product.prod_no=v_inv.prod_no where datediff(day,v_inv.inv_date,getdate())<30)a

试试
[解决办法]
select count (*) from product left join v_inv on product.prod_no=v_inv.prod_no
where datediff(day,v_inv.inv_date,getdate())<30

where 是连接后筛选符合条件的记录，顺序不一样，可以试试：

select count (*)
from product left join v_inv on product.prod_no=v_inv.prod_no
and datediff(day,v_inv.inv_date,getdate())<30
[解决办法]
where datediff(day,v_inv.inv_date,getdate())<30限制了最终输出条数，如果条件全写在ON里就是只会多不会少。
[解决办法]

SQL code

select  count(*)from    product        left join ( select  prod_no                    from    v_inv                    where   datediff(day, v_inv.inv_date, getdate()) < 30                  ) v_inv on product.prod_no = v_inv.prod_no--作一个子查询再连接，就可以了