基於自定義計算字段篩選行

Question

我想篩選使用WHERE條件返回的行。問題是MySQL不允許我在WHERE子句中使用自定義列名。它給出了語法錯誤。

SELECT jobs.*, 

     CASE 
      WHEN job_status = 'submitted'   THEN ((2 * 86400) - ($mktime - job_timestamp))/86400 
      WHEN job_status = 'agreement_pending' THEN ((1 * 86400) - ($mktime - job_tstamp_agpending))/86400 
      WHEN job_status = 'payment_complete' THEN ((job_duration * 86400) - ($mktime - job_tstamp_pcomplete))/86400 
      ELSE 1000000 
     END AS sla 

FROM jobs 
WHERE sla < 0 

是否有可能仍然過濾行了WHERE sla < 0條件？

Answer 1

MySQL有一個特殊的擴展，你可以使用having條款做到這一點：

SELECT jobs.*, 

     CASE 
      WHEN job_status = 'submitted'   THEN ((2 * 86400) - ($mktime - job_timestamp))/86400 
      WHEN job_status = 'agreement_pending' THEN ((1 * 86400) - ($mktime - job_tstamp_agpending))/86400 
      WHEN job_status = 'payment_complete' THEN ((job_duration * 86400) - ($mktime - job_tstamp_pcomplete))/86400 
      ELSE 1000000 
     END AS sla 

FROM jobs 
HAVING sla < 0; 

這不是在其他數據庫的支持。

Answer 2

所有的RDBMS的有效解決方案使用派生表：

SELECT * 
FROM 
    (
    SELECT jobs.*, 
     CASE 
      WHEN job_status = 'submitted'   THEN ((2 * 86400) - ($mktime - job_timestamp))/86400 
      WHEN job_status = 'agreement_pending' THEN ((1 * 86400) - ($mktime - job_tstamp_agpending))/86400 
      WHEN job_status = 'payment_complete' THEN ((job_duration * 86400) - ($mktime - job_tstamp_pcomplete))/86400 
      ELSE 1000000 
     END AS sla 
    FROM jobs 
) AS dt 
WHERE sla < 0