Oracle/MySQL 從dual/help_topic中獲取多行數據

背景與意義：某學長寫出了一段又臭又長的代碼，引發了我的思考

獲取12個月的月份：

month

1

2

3

4

5

6

7

8

9

10

11

12

正常操作：

SELECT MONTH 
FROM
  (
  SELECT
    1 MONTH UNION ALL
  SELECT
    2 MONTH UNION ALL
  SELECT
    3 MONTH UNION ALL
  SELECT
    4 MONTH UNION ALL
  SELECT
    5 MONTH UNION ALL
  SELECT
    6 MONTH UNION ALL
  SELECT
    7 MONTH UNION ALL
  SELECT
    8 MONTH UNION ALL
  SELECT
    9 MONTH UNION ALL
  SELECT
    10 MONTH UNION ALL
  SELECT
    11 MONTH UNION ALL
  SELECT
  12 MONTH 
  ) months;

因爲實在是太長了，希望能簡短一點，因此調用MySQL的help_topic進行改寫：

MySQL版本

SELECT
	substring_index( substring_index( m_str, m_split, help_topic_id + 1 ), m_split,- 1 ) AS MONTH 
FROM
	mysql.help_topic,
	( SELECT '1,2,3,4,5,6,7,8,9,10,11,12' AS m_str, ',' AS m_split FROM DUAL ) AS m_tbl 
WHERE
	help_topic_id < ( length( m_tbl.m_str ) - length( REPLACE ( m_tbl.m_str, m_split, '' )) + 1 );

使用了拆分字符串的思想，其中m_str就是每行需要顯示的數值，m_split 是分隔符。

輸出結果

Oracle版：

SELECT
	ROWNUM MONTH 
FROM
	DUAL CONNECT BY LEVEL <= 12;

Oracle輸出結果