MySQL 变量作为带有其他表元素的子查询的结果

Question

我有一个有效的查询:

SELECT count(*),
       AVG(
             (SELECT sum(order_fills.quant * order_fills.price)
              FROM order_fills
              WHERE `type`='closed'
                AND order_fills.transaction_id = transactions.id) -
             (SELECT sum(order_fills.quant * order_fills.price)
              FROM order_fills
              WHERE `type`='opened'
                AND order_fills.transaction_id = transactions.id)) AS avgProfit,
       SUM(
             (SELECT sum(order_fills.quant * order_fills.price)
              FROM order_fills
              WHERE `type`='closed'
                AND order_fills.transaction_id = transactions.id)) AS volume,
FROM transactions
WHERE transactions.status='closed';

为了使代码更具可读性，我希望将该查询的某些部分设置为变量。

我希望实现类似的目标(它不起作用，返回null):

SET @transSells =
          (SELECT sum(order_fills.quant * order_fills.price)
           FROM order_fills
           WHERE type='closed'
             AND order_fills.transaction_id = transactions.id);


SET @transBuys =
          (SELECT sum(order_fills.quant * order_fills.price)
           FROM order_fills
           WHERE type='opened'
             AND order_fills.transaction_id = transactions.id);


SELECT avg(@transSells - @transBuys) AS `avgProfit`,
       sum(@transSells + @transBuys) AS `volume`
FROM transactions WHERE transactions.status='closed'

我认为问题可能是子查询WHERE语句的一部分是主查询和另一个表(transactions.id)的结果:

AND order_fills.transaction_id = transactions.id

Answer 1

变量查询不起作用的原因是变量定义的 WHERE 子句中的 transactions.id 未定义 (NULL) >)，因此查询不会返回任何结果。

我认为您可以通过使用条件聚合将变量放入派生表中来简化查询，然后JOIN到:

SELECT AVG(transSells - transBuys) AS avgProfit,
       SUM(transSells + transBuys) AS volume
FROM transactions
JOIN (SELECT transaction_id,
             SUM(CASE WHEN type='closed' THEN quant * price ELSE 0 END) AS transSells,
             SUM(CASE WHEN type='opened' THEN quant * price ELSE 0 END) AS transBuys
      FROM order_fills) o ON o.transaction_id = transactions.id
WHERE status = 'closed'