Mysql 在联接查询中将列设置为null或空

订单表

产品表

使用SELECT*FROM order o LEFT在o.order\u id=p.order\u id上连接产品p

结果

 PRODUCT ID | ORDER ID|ORDER_DT
 1          | 1       |12-12-2016
 2          | 1       |12-12-2016
 3          | 1       |12-12-2016

但是我需要有这样的结果，注意到我仍然有相同数量的行，但是表列被清空的顺序是重复的

PRODUCT ID | ORDER ID|ORDER_DT
     1     | 1       |12-12-2016
     2     |         |
     3     |         |

---

您可以使用临时变量执行此操作，例如：

SELECT product_id, orders.order_id, order_dt, IF(@previous = orders.order_id, 0, orders.order_id) AS oid, @previous:= orders.order_id
FROM orders
LEFT JOIN products
ON orders.order_id = products.order_id,
(SELECT @previous := -1) a;

并将其包装到另一个SELECT查询中以获取所需的列，例如：

SELECT r.product_id, IF(r.oid = 0, '', r.oid) AS order_id, r.order_dt 
FROM (
    SELECT product_id, orders.order_id, order_dt, IF(@previous = orders.order_id, 0, orders.order_id) AS oid, @previous:= orders.order_id
    FROM orders
    LEFT JOIN products
    ON orders.order_id = products.order_id,
    (SELECT @previous := -1) a
    ) r;

---

以下是.

这更像是在表示层而不是在数据库中进行的显示操作。在join子句中缺少ON条件，因此得到的是笛卡尔积。或者，如果数据库中有多个订单，您也会这样做！抱歉更新了问题@LordPeter

SELECT product_id, orders.order_id, order_dt, IF(@previous = orders.order_id, 0, orders.order_id) AS oid, @previous:= orders.order_id
FROM orders
LEFT JOIN products
ON orders.order_id = products.order_id,
(SELECT @previous := -1) a;

SELECT r.product_id, IF(r.oid = 0, '', r.oid) AS order_id, r.order_dt 
FROM (
    SELECT product_id, orders.order_id, order_dt, IF(@previous = orders.order_id, 0, orders.order_id) AS oid, @previous:= orders.order_id
    FROM orders
    LEFT JOIN products
    ON orders.order_id = products.order_id,
    (SELECT @previous := -1) a
    ) r;