这个错误通常是因为在一个标量子查询(Scalar Subquery)中返回了多个结果。例如,在下面的例子中:
SELECT name, (SELECT COUNT(*) FROM orders WHERE customer_id = customers.id) AS order_count FROM customers
如果有多个订单与同一个客户 ID 关联,则标量子查询将返回多个结果,并且无法将它们插入到一个标量表达式中。为了解决这个问题,可以使用一个聚合函数(如 SUM()、COUNT())或 LIMIT 语句来限制返回的结果数量。例如,你可以将上面的查询改写为:
SELECT name, (SELECT COUNT(*) FROM orders WHERE customer_id = customers.id LIMIT 1) AS order_count FROM customers
或者:
SELECT name, COUNT(*) as order_count FROM customers JOIN orders ON customers.id = orders.customer_id GROUP BY name