我如何在此查詢中使用子查詢的結果？

Question

例如，我有SQL-查詢：

SELECT
    (SELECT value->>"$[0].value" 
        FROM leads_fields 
        WHERE leads_fields.field_id = 244991 AND leads_fields.lead_id = leads.external_id)
        as 'coefRL',
    (SELECT value->>"$[0].value" 
        FROM leads_fields 
        WHERE leads_fields.field_id = 244943 AND leads_fields.lead_id = leads.external_id)
        as 'coefWF'

FROM leads

為了獲得這些值的差異，我需要使用第3個子查詢來復制這些請求嗎？

(
    (SELECT value->>"$[0].value" 
     FROM leads_fields 
     WHERE leads_fields.field_id = 244991 AND leads_fields.lead_id = leads.external_id)
        -
     (SELECT value->>"$[0].value" 
      FROM leads_fields 
      WHERE leads_fields.field_id = 244943 AND leads_fields.lead_id = leads.external_id)
) as 'difference'

還是有一些更實用的方法，例如(coefRL - coefWF) ？

Answer 1

這是沒有第三個子查詢的一種方法。

SELECT coefRL,
  coefWF,
  coefRL-coefWF difference
FROM
  (SELECT
    (SELECT value->>"$[0].value"
    FROM leads_fields
    WHERE leads_fields.field_id = 244991
    AND leads_fields.lead_id    = leads.external_id
    ) AS 'coefRL',
    (SELECT value->>"$[0].value"
    FROM leads_fields
    WHERE leads_fields.field_id = 244943
    AND leads_fields.lead_id    = leads.external_id
    ) AS 'coefWF'
  FROM leads
  )