如何对 MySql 中返回的最顶层行执行计算？答案

【问题标题】：How to perform calculation on the topmost returned row in MySql?如何对 MySql 中返回的最顶层行执行计算？
【发布时间】：2020-08-19 14:51:03
【问题描述】：

我编写了以下查询，其中使用order by 子句返回行：

SELECT host.hostname,
       cb.orderId,
       cb.status,
       cb.chefbook_name AS Heading,
       (SELECT log_message
        FROM   chefbookrun rr
        WHERE  rr.chefbook_id = c.id
        ORDER  BY id DESC 
        LIMIT  1)      AS Detail 
FROM   host
       INNER JOIN host_infl hif
               ON host.vc_server_id = hif.vc_server_id
       INNER JOIN chefcookbook cb
               ON hif.id = cb.host_id
WHERE  host.hostname REGEXP 'abc'
ORDER  BY cb.orderId;

有几种不同类型的输出，如下所示：

hostname    orderId status  Heading         Detail
abc.com     3       30      HeadingA        Details1
abc.com     5       40      HeadingB        Details2
... more rows

hostname    orderId status  Heading         Detail
cde.com     3       40      HeadingA        Details1
cde.com     5       30      HeadingB        Details2
... more rows

hostname    orderId status  Heading         Detail
efg.com     3       50      HeadingA        Details1
efg.com     5       30      HeadingB        Details2
... more rows

我想对第一个返回行的status 执行如下计算：

1) If `status is 30` then show status as `Finished`. Also `Heading` and `Detail` should be empty.

2) If `status is 40` or `status is 50` then show status as `Error`. `Heading` and `Detail` should show as such.

想要的输出如下：

hostname    orderId status     Heading          Detail
abc.com     3       Finished

hostname    orderId status  Heading         Detail
cde.com     3       Error   HeadingA        Details1

hostname    orderId status  Heading         Detail
efg.com     3       Error   HeadingA        Details1

【问题讨论】：

标签： mysql join select sql-order-by inner-join

【解决方案1】：

SELECT host.hostname,orderId,
CASE WHEN status = 30 THEN 'Finished' ELSE 'Error' END 'status',
CASE WHEN status = 30 THEN '' ELSE Heading END 'Heading',
CASE WHEN status = 30 THEN '' ELSE Detail END 'Detail'
FROM
(
     SELECT host.hostname,
     cb.orderId,
     cb.status,
     cb.chefbook_name AS Heading,
     (
         SELECT log_message
         FROM   chefbookrun rr
         WHERE  rr.chefbook_id = c.id
         ORDER  BY id DESC 
         LIMIT 1 
     ) AS Detail 
     FROM host
     INNER JOIN host_infl hif ON host.vc_server_id = hif.vc_server_id
     INNER JOIN chefcookbook cb ON hif.id = cb.host_id
     WHERE  host.hostname REGEXP 'abc'
     ORDER  BY cb.orderId
) AS temp
GROUP BY hostname;

【讨论】：