Question

因此有两个问题:

SELECT CurrencyRateID, Rate, Markup
FROM currency_rate 
WHERE CurrencyID = (SELECT CurrencyID FROM currency WHERE BaseCurr = 1) **A** AND
DateTime = (SELECT MAX(DateTime) 
            FROM currency_rate 
            WHERE CurrencyID = **B**)

Why does this query give me an Error Code: 1054. Unknown column CurrercyID in field list
How can I use the result of the first SELECT Statement A in B

Answer 1

希望您的<代码>currency_rate table 有一个独特的识别栏。

如果是的话,你可以将问询A和B合并为WHERE条款的单一询问。

SELECT
  CurrencyRateID, Rate, Markup
FROM
  currency_rate 
WHERE
  uniqueID = (
              SELECT uniqueID
              FROM   currency_rate
              WHERE  CurrencyID = (SELECT CurrencyID FROM currency WHERE BaseCurr = 1)
              ORDER BY DateTime DESC LIMIT 1
             )

否则,你可能不得不重复......

SELECT CurrencyRateID, Rate, Markup
FROM currency_rate 
WHERE CurrencyID = (SELECT CurrencyID FROM currency WHERE BaseCurr = 1)
AND DateTime = (SELECT MAX(DateTime) 
            FROM currency_rate 
            WHERE CurrencyID = (SELECT CurrencyID FROM currency WHERE BaseCurr = 1))

注:尽管守则重复,但MySQL可能注意到重新计算,只是一次处理。

http://www.un.org。

I just noticed that you could simplify the first option to just use the sub-query...

  SELECT CurrencyRateID, Rate, Markup
  FROM   currency_rate
  WHERE  CurrencyID = (SELECT CurrencyID FROM currency WHERE BaseCurr = 1)
  ORDER BY DateTime DESC LIMIT 1

Answer 2

货币信用卡一栏以货币表格的形式存在,因此,在条款中可以使用报表A。

SELECT CurrencyRateID, Rate, Markup
FROM currency_rate cr inner join 
   (SELECT CurrencyID FROM currency WHERE BaseCurr = 1) **A**
   ON cr.CurrencyID = a.CurrencyID 
WHERE cr.DateTime = (SELECT MAX(DateTime) 
            FROM currency_rate 
            WHERE CurrencyID = A.CurrencyID)

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