在Postgresql的where子句中使用Alias列

Question

我有这样的查询：

SELECT
    jobs.*, 
    (
        CASE
            WHEN lead_informations.state IS NOT NULL THEN lead_informations.state
            ELSE 'NEW'
        END
    ) AS lead_state
FROM
    jobs
    LEFT JOIN lead_informations ON
        lead_informations.job_id = jobs.id
        AND
        lead_informations.mechanic_id = 3
WHERE
    lead_state = 'NEW'

出现以下错误：

PGError: ERROR:  column "lead_state" does not exist
LINE 1: ...s.id AND lead_informations.mechanic_id = 3 WHERE (lead_state...

在MySql中，这是有效的，但显然在Postgresql中是无效的。据我所知，原因是SELECT查询部分的评估晚于该WHERE部分。是否有解决此问题的常用解决方法？

Answer 1

正如您所经历的那样，MySQL的支持是非标准的。正确的方法是重新打印SELECT子句中使用的相同表达式：

SELECT
    jobs.*, 
    CASE 
         WHEN lead_informations.state IS NOT NULL THEN lead_informations.state 
         ELSE 'NEW' 
    END AS lead_state
FROM
    jobs
    LEFT JOIN lead_informations ON
        lead_informations.job_id = jobs.id
        AND
        lead_informations.mechanic_id = 3
WHERE
    lead_informations.state IS NULL