数据库中主查询中的子查询问题

USE sql_invoicing;

SELECT
client_id,
name,
/*(SELECT SUM(invoice_total)
FROM invoices
GROUP BY client_id)
为啥这个会报错？
*/
(SELECT SUM(invoice_total)
FROM invoices
WHERE client_id = c.client_id) AS total_sales,
(SELECT AVG(invoice_total)
FROM invoices) AS average,
(SELECT total_sales) - (SELECT average) AS difference
FROM clients c

你把最关键的信息没有截图出来。。最后一幅图第一行记录的message那里

不能用select 查询列值用嵌套查询用group by关键词,查出来可能会不止一条,你加个limit 1 试试看