t1表有数据，t2表没数据 ，sql返回的结果也是没数据 ，怎么处理，可以让有数据返回

如果 t2 表中没有数据，那么 CROSS JOIN 之后的结果集就会为空，因此返回的结果也会是空。为了让有数据返回，可以将 CROSS JOIN 改成 LEFT JOIN，这样即使 t2 表中没有数据，t1 表中的数据也会被返回。修改后的 SQL 查询语句如下：

SELECT t1.*, t2.*
FROM (
    SELECT COUNT(1) AS order_quantity,
           SUM(IFNULL(amounts_payable, 0)) AS order_amount,
           (ROUND(SUM(IFNULL(amounts_payable, 0)) / COUNT(1), 2)) AS order_average_unit_price,
           COUNT(IF(product_type = 1, 1, NULL)) AS experience_product_order,
           SUM(IF(product_type = 1, IFNULL(amounts_payable, 0), NULL)) AS experience_order_amount,
           COUNT(IF(product_type = 2, 1, NULL)) AS formal_product_order,
           SUM(IF(product_type = 2, IFNULL(amounts_payable, 0), NULL)) AS formal_order_amount,
           "1" AS param
    FROM `order_management`
    WHERE order_status = 1
      AND DATE_FORMAT(create_time, '%Y-%m-%d') = DATE_FORMAT(NOW(), '%Y-%m-%d')
    GROUP BY DATE_FORMAT(create_time, '%Y-%m-%d')
) t1
LEFT JOIN (
    SELECT COUNT(IF(student_type = 0, 1, NULL)) AS registered_student_num,
           COUNT(IF(student_type = 1, 1, NULL)) AS experience_student_num,
           COUNT(IF(student_type = 2, 1, NULL)) AS regular_student,
           "1" AS param
    FROM student_record
    WHERE DATE_FORMAT(create_time, '%Y-%m-%d') = DATE_FORMAT(NOW(), '%Y-%m-%d')
    GROUP BY DATE_FORMAT(create_time, '%Y-%m-%d')
) t2
ON t1.param = t2.param

这样修改后，即使 t2 表中没有数据，t1 表中的数据也会被返回，同时 t2 表中的列值都会被设置为 NULL。