Question

我有以下查询：

SELECT  t1.id,
        t1.collection_date,
        t2.last_order_date,
        t2.hasOrders
FROM job_details AS t1
    LEFT JOIN
        (
        SELECT  job_id, 
                MAX(date) AS last_order_date,
                (CASE COALESCE(MAX(date), '0000-01-01') WHEN '0000-01-01' THEN 'false' ELSE 'true' END) AS hasOrders
        FROM sales_orders
        GROUP BY job_id
        )
    AS t2
    ON t2.job_id = t1.id

我期望t2.hasOrders的{{1}}部分之后的true或false或CASE，但我一直得到LEFT JOIN或将true设为demonstrated here

我知道NULL将始终返回有效日期或默认列值t2.date。

这是我查询的简化版本，因为0000-00-00正在查询的表是一个临时表。

为什么LEFT JOIN的{{1}}值不返回我的预期结果？

谢谢。

编辑1

链接到演示-删除了查询的NULL部分

编辑2

预期结果：

t2.date

实际结果：

where

编辑3

已将上述查询更新为在查询的id collection_date last_order_date hasOrders 1001 2019-02-07 2019-02-01 true 1002 2019-02-09 0000-01-01 false部分中包含id collection_date last_order_date hasOrders 1001 2019-02-07 2019-02-01 true 1002 2019-02-09 NULL NULL，并根据建议将group by更改为left join。仍然无法正常工作。

Answer 1

您需要将CASE WHEN移到外部查询，因为如果sales_orders中没有行，则内部CASE WHEN将永远不会执行

SELECT  t1.id,
    t1.collection_date,
    COALESCE(t2.last_order_date, '0001-01-01') order_date,
    CASE COALESCE(t2.last_order_date, '0001-01-01') 
      WHEN '0001-01-01' THEN 'false' 
      ELSE 'true' 
    END flag
FROM job_details AS t1
LEFT JOIN
  (SELECT  job_id, MAX(date) AS last_order_date
   FROM sales_orders
   GROUP BY job_id
  ) AS t2
 ON t2.job_id = t1.id

Answer 2

我认为，通过将“ Max”和“ group by”移到外部语句，逻辑至少应该可以正常工作（如果您也要计算该列，则需要添加“ Coalesce”语句）。不确定性能如何，但是我首先会看看结果是否还可以。希望语法很好，还没有测试。

SELECT  t1.id,
        t1.collection_date,
        MAX(t2.last_order_date)
FROM job_details AS t1
        LEFT JOIN
        (
        SELECT  job_id, 
                last_order_date
        FROM sales_orders
        )
    AS t2
    ON t2.job_id = t1.id
GROUP BY t1.id

Answer 3

您必须输入t1.collection_date <= '2019-02-09'，因为job_id 1002日期是'2019-02-09'

SELECT  t1.id,
        t1.collection_date,
        t2.order_date,
        t2.hasOrders
FROM job_details AS t1
    LEFT JOIN
        (
        SELECT  job_id, 
                MAX(date) AS order_date,
                (CASE COALESCE(MAX(date), '0000-00-00') WHEN '0000-00-00' THEN 'false' ELSE 'true' END) AS hasOrders
        FROM sales_orders group by job_id
        )
    AS t2
    ON t2.job_id = t1.id
WHERE t1.collection_date <= '2019-02-09'

MySql Case不返回NULL的期望值

3 个答案: