MySQL查询中使用union的技巧和注意事项
把一张表的一条记录（通过不同的条件约束）变成两条记录时，一般会使用union来实现。但是需要注意的是，要给拆出来的两张表第一个字段附上不重复的ID，不然就会踩到坑。SELECT sum(m_fetch_num), sum(m_return_num) FROM
(
(
SELECT concat('0_',order_id) as order_id,
a.order_fetch_time,a.order_fetch_distanc,
a.m_fetch_num,0 as m_return_num
FROM dwm_flash_order_fetch_return as a
WHERE m_fetch_num>0
)
UNION
(
SELECT concat('1_',order_id) as order_id,
0 as order_fetch_time,0 as order_fetch_distanc,
0 as m_fetch_num,a.m_return_num
FROM dwm_flash_order_fetch_return as a
WHERE m_return_num>0
)
) x
页:
[1]