示例：
SELECT COUNT(id) AS total_orders,
( ( SELECT COUNT(id) FROM orders WHERE status = 'Closed' ) / total_orders * 100) AS percent_closed_orders
FROM orders
问题：如何在子查询中使用total_orders？我得到一个错误"；"字段列表"中的未知列"total_orders"；。
SELECT COUNT(id) AS total_orders,
SUM( status = 'Closed' ) / COUNT(id) * 100 AS percent_closed_orders
FROM orders
如果id不能为NULL(例如它是主键(，则可以使用COUNT(*)来计算总行数。
不能在定义列别名的同一SELECT语句中使用列别名。但是，如果在外部SELECT语句中定义了别名，则可以在随后的SELECT语句中使用该别名。
在您的示例中，您需要将内部SELECT语句移动到子查询中，以便能够使用total_orders别名：
SELECT COUNT(id) AS total_orders,
( ( SELECT COUNT(id) FROM orders WHERE status = 'Closed' ) / (SELECT total_orders FROM (SELECT COUNT(id) AS total_orders FROM orders) AS x) * 100) AS percent_closed_orders
FROM orders