求交易总额的最大值两种方法：
SELECT c.merchant_no,SUM(c.transaction_amount) AS tt FROM mer_detail c WHERE c.created_date >"2020-09-01" GROUP BY c.merchant_no ORDER BY tt DESC LIMIT 1; SELECT MAX(dd.tt) from (SELECT c.merchant_no,SUM(c.transaction_amount) AS tt FROM mer_detail c WHERE c.created_date >"2020-09-01" GROUP BY c.merchant_no ) as dd ;
查询重复数据：
SELECT * FROM clear_mer_detail c GROUP BY c.transaction_no HAVING COUNT(c.transaction_no)>1 ORDER BY c.created_date DES
原文：https://www.cnblogs.com/pandaly/p/13638894.html