关于 order by 子句里面用到 case when 的例子

SELECT * FROM (
SELECT 'a' AS a , 1 AS b FROM dual
UNION ALL
SELECT 'a' AS a , 2 AS b FROM dual
UNION ALL
SELECT 'b' AS a , 2 AS b FROM dual
UNION ALL
SELECT 'b' AS a , 1 AS b FROM dual
) a
ORDER BY (CASE WHEN a.a = 'a' THEN 't' ELSE 'd' END ) + a.b

这里很奇怪，去掉红色的部分就可以正常运行。

此例应该如何改才能排序出
b,1
b,2
a,1
a,2
1 要求在order by 子句里面用到 case when
2 除了 order by a desc , b 这个答案。



[最优解释]
'a'+1 当然报错

+a.b改成
[其他解释]
字符和数字怎么相加！也没意义吧,改成数字就可以了！

SELECT * FROM (
  SELECT 'a' AS a , 1 AS b FROM dual
  UNION ALL
  SELECT 'a' AS a , 2 AS b FROM dual
  UNION ALL
  SELECT 'b' AS a , 2 AS b FROM dual
  UNION ALL
  SELECT 'b' AS a , 1 AS b FROM dual
) a
ORDER BY (CASE WHEN a.a = 'a' THEN 2 ELSE 1 END)+a.b