如果您也指定了输出值，那么就很容易理解了。对于您的问题，是的，您可以使用case在一个查询中实现转换

SELECT
  center,
  account,
  SUM(expenses)/SUM(users) AS result
FROM
 (
  SELECT
    t2.center AS center,
    t1.users AS users,
    CASE
     WHEN t2.currency = 'USD' THEN (t2.expenses*70)
     WHEN t2.currency = 'MYR' THEN (t2.expenses*17)
     WHEN t2.currency = 'AUD' THEN (t2.expenses*50)
     WHEN t2.currency = 'INR' THEN t2.expenses
     ELSE 'invalid'
    END AS expenses,
    t2.currency AS currency,
    t2.account AS account
  FROM
  table1 t1 
  JOIN 
  table2 t2 
  ON(t1.center=t2.center)
 ) joined_data
GROUP BY center,account
ORDER BY center,account

我没有测试这个，但它应该工作。在最坏的情况下，它可能会抛出一个小的语法错误，您应该能够处理它。：）