要想理解递归，先得理解递归

这次是SQL递归

1 WITH Seq (num) AS  
2 (SELECT 1
3 UNION ALL  
4 SELECT num + 1
5 FROM Seq  
6 WHERE num <= 50 
7 )  
8 SELECT num
9 FROM Seq

比较难以理解，正如标题：要想理解递归，先得理解递归！