Merchant table

The column name annotation
id Merchant id
city city

Transaction flow statement (transaction_flow)

The column name annotation
m_id Merchant id
amount Transaction amount
time Trading hours

Please write SQL to query the monthly sales performance of each city.

SELECT
  m.city, YEAR(t.time), MONTH(t.time), SUM(t.amount)
FROM
  transaction_flow t
  LEFT JOIN merchant m ON t.m_id = m.id
GROUP BY m.city, YEAR(t.time), MONTH(t.time);
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