group_concat，substring_index

示例1

• 连接起来的字段如果是int型，一定要转换成char再拼起来

  select group_concat(CAST(id as char)) from t_dep;   -- 返回逗号隔开的串
  

select year,month GROUP_CONCAT(volumn) from magazine group by year,month order by year desc, month desc;  

year    month   GROUP_CONCAT(volumn)
2010    12  1,2,3,4
2010    11  1,2,3,4,5

示例2

SELECT 
ticket_id, 
GROUP_CONCAT(DISTINCT odd_id ORDER BY odd_id ASC) as oddsconcat 
FROM ticket_odds 
GROUP BY ticket_id 
HAVING oddsconcat = .....

示例3

在一个表ta中数据为：

+----+------+
| id | name |
+----+------+
| 1 | a    |
| 1 | b    |
| 1 | c    |
| 1 | d    |
| 2 | a    |
| 2 | b    |
| 2 | c    |
| 3 | d    |
+----+------+

从里面取数据要求完成如下效果:

+----+--------+
| id | name    |
+----+--------+
| 1 | a,b,c,d |
| 2 | a,b,c    |
| 3 | d          |
+----+--------+

SELECT `id`,GROUP_CONCAT(`name` ORDER BY `name` DESC SEPARATOR ',') AS name FROM `ta` GROUP BY `id`;  

示例4

• 用法：SUBSTRING_INDEX(str,delim,count)

  返回字符串 str 中在第 count 个出现的分隔符 delim 之前的子串。如果 count 是一个正数，返回从最后的(从左边开始计数)分隔符到左边所有字符。如果 count 是负数，返回从最后的(从右边开始计数)分隔符到右边所有字符

  mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2);
        -> 'www.mysql'
  mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', -2);
        -> 'mysql.com'
  

  -- 在上面一个例子中，我们使用“,”来分隔数据，取前面2个，就可以这样写：
  SELECT `id`,SUBSTRING_INDEX(GROUP_CONCAT(`name` ORDER BY `name` DESC SEPARATOR ','),',',2) AS `name` FROM `ta` GROUP BY `id`;
  

  In general, to select the N-th value from a string VALUES that contains values separated by delimiter DELIM, you have to use:

  SUBSTRING_INDEX( SUBSTRING_INDEX( VALUES, DELIM, N ), DELIM, -1 )