在Postgresql的where子句中使用Alias列

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中无效。据我所知，原因是SELECT查询部分的评估晚于该WHERE部分。有没有解决此问题的常用方法？

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

我在同一个问题上苦苦挣扎，我认为“ mysql语法是非标准的”不是有效的参数。PostgreSQL还添加了方便的非标准扩展名，例如“ INSERT ... RETURNING ...”，以便在插入后获得自动ID。同样，重复大型查询也不是一个很好的解决方案。

但是，我发现WITH语句非常有帮助。它在查询中创建了一个临时视图，您可以像使用普通表一样使用它。我不确定我是否正确重写了JOIN，但总的来说，它应该像这样工作：

WITH jobs_refined AS (
    SELECT
        jobs.*,
        (SELECT 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
)
SELECT *
FROM jobs_refined
WHERE lead_state = 'NEW'

您可能需要在where子句中复制case语句，或者我的偏好是执行以下操作：

SELECT *
FROM (
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
) q1
WHERE (lead_state = 'NEW')

我相信常见的解决方案是使用内部SELECT进行计算（在这种情况下为CASE语句），以便在执行到该外部查询时，整个外部查询都可以使用内部SELECT的结果。否则，将首先评估WHERE子句，并且对SELECT子句一无所知。

我在这样的地方使用别名。（内部查询）。

Select "Vendors"."VendorId", "Vendors"."Name","Result"."Total" 
From (Select "Trans"."VendorId", ("Trans"."A"+"Trans"."B"+"Trans"."C")    AS "Total"
        FROM "Trans"
    WHERE "Trans"."Year"=2014                                                
    ) As "Result"
JOIN "Vendors" ON "Result"."VendorId"="Vendors"."VendorId" 
WHERE "Vendors"."Class"='I' AND "Result"."Total" > 200

SELECT "tab_1"."BirthDate", "tab_1"."col_1" FROM (
   SELECT BirthDate, DATEADD(year, 18, BirthDate) AS "col_1" FROM Employees
) AS "tab_1"
WHERE "tab_1"."col_1" >= '2000-12-31';