把两行清楚地连在一起

我不明白你为什么不做更简单的事:

SELECT num AS num1, num AS num2 FROM x

或者,如果您确实希望将其作为字符串:

SELECT CONCAT(num, ',', num) AS num FROM x

SELECT CONCAT(T1.num, ',', T2.num) AS num
FROM x AS T1
JOIN x AS T2
ON T1.num = T2.num

如果您需要来自不同行的字段,例如“1,2”、“2,3”等,则更有意义。。。也许你提出问题时把事情简单化了?您只是想学习如何使用连接语法吗?

您可以删除所有其他内容,只需在select语句中列出该列两次。所以在这种情况下,应该是:

Select x.num as First_Num, x.num as Second_num from x

SELECT CONCAT(num, ',', num) AS num FROM x