Mysql 按值之和限制查询

我有一个mysql表，transactions，带有id、transactionType、quantity、price等字段。我想通过一列的累积和来限制记录。因此，我想提取所有事务，直到累计数量达到我的变量尝试这种方法

SET @qsum := 0; 
SELECT * 
FROM (
  SELECT *, (@qsum := @qsum + quantity) AS cumulative_quantity 
  FROM transactions
  WHERE transactionType = 'buy'
   AND typeID = 10
   AND cumulative_quantity <= 50
  ORDER BY id DESC
) transaction 

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SET @qsum := 0; 
SELECT * 
FROM (
  SELECT *, (@qsum := @qsum + quantity) AS cumulative_quantity 
  FROM transactions
  WHERE transactionType = 'buy'
   AND typeID = 10
   AND cumulative_quantity <= 50
  ORDER BY id DESC
) transaction