MySQL 字串拼接詳解

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在Mysql 資料庫中存在兩種字串串連操作.具體操作如下
一. 文法:
   1. CONCAT(string1,string2,…)   說明 : string1,string2代表字串,concat函數在連接字串的時候，只要其中一個是NULL,那麼將返回NULL
   例1:
   
   例2:

  
  2. CONCAT_WS(separator,str1,str2,...)
     說明 : string1,string2代表字串,concat_ws 代表 concat with separator,第一個參數是其它參數的分隔字元。分隔字元的位置放在要串連的兩個字串之間。分隔字元可以是一個字串，也可以是其它參數。如果分隔字元為 NULL，則結果為 NULL。函數會忽略任何分隔字元參數後的 NULL 值。
     舉例1:
     mysql> select concat_ws(‘#‘,‘dbdh=‘,‘NorthEastTrcoon‘,null) AS dbdh_name_three;
+-----------------------+
| dbdh_name_three       |
+-----------------------+
| dbdh=#NorthEastTrcoon |
+-----------------------+
1 row in set (0.00 sec)
    例2:
  mysql> select concat_ws(null,‘dbdh=‘,‘NorthEastTrcoon‘,null) AS dbdh_name_fourth;
+------------------+
| dbdh_name_fourth |
+------------------+
| NULL             |
+------------------+
1 row in set (0.00 sec)
    例3:
       mysql> select concat_ws(‘*‘,‘dbdh=‘,‘NorthEastTrcoon‘,null) AS dbdh_name_fifth;
+-----------------------+
| dbdh_name_fifth       |
+-----------------------+
| dbdh=*NorthEastTrcoon |
+-----------------------+
1 row in set (0.00 sec)
3. MySQL中group_concat函數
完整的文法如下：
group_concat([DISTINCT] 要串連的欄位 [Order BY ASC/DESC 排序欄位] [Separator ‘分隔字元‘])
基本查詢
mysql> select * from stu1;
+------+------+
| id| name |
+------+------+
|1 | 10|
|1 | 20|
|1 | 20|
|2 | 20|
|3 | 200   |
|3 | 500   |
+------+------+
6 rows in set (0.00 sec)

以id分組，把name欄位的值列印在一行，逗號分隔(預設)

mysql> select id,group_concat(name) from aa group by id;
+------+--------------------+
| id| group_concat(name) |
+------+--------------------+
|1 | 10,20,20|
|2 | 20 |
|3 | 200,500|
+------+--------------------+
3 rows in set (0.00 sec)

以id分組，把name欄位的值列印在一行，分號分隔

mysql> select id,group_concat(name separator ‘;‘) from aa group by id;
+------+----------------------------------+
| id| group_concat(name separator ‘;‘) |
+------+----------------------------------+
|1 | 10;20;20 |
|2 | 20|
|3 | 200;500   |
+------+----------------------------------+
3 rows in set (0.00 sec)

以id分組，把去冗餘的name欄位的值列印在一行，
逗號分隔
mysql> select id,group_concat(distinct name) from aa group by id;
+------+-----------------------------+
| id| group_concat(distinct name) |
+------+-----------------------------+
|1 | 10,20|
|2 | 20   |
|3 | 200,500 |
+------+-----------------------------+
3 rows in set (0.00 sec)
以id分組，把name欄位的值列印在一行，逗號分隔，以name排倒序
mysql> select id,group_concat(name order by name desc) from aa group by id;
+------+---------------------------------------+
| id| group_concat(name order by name desc) |
+------+---------------------------------------+
|1 | 20,20,10   |
|2 | 20|
|3 | 500,200|
+------+---------------------------------------+
3 rows in set (0.00 sec)
還有一個簡單的串連方式為: ||
mysql> select *  from student;
+----+------+-------+----------+------------+
| id | age  | score | name     | birth      |
+----+------+-------+----------+------------+
|  1 |   23 |    78 | 李四     | 2017-10-10 |
|  2 |   24 |    78 | zhangsan | 2017-10-10 |
|  3 |   25 |    99 | 王五     | 2016-05-17 |
+----+------+-------+----------+------------+
3 rows in set (0.00 sec)
mysql> select id+999,name,name+99,name+‘999‘ from student;
+--------+----------+---------+------------+
| id+999 | name     | name+99 | name+‘999‘ |
+--------+----------+---------+------------+
|   1000 | 李四     |      99 |        999 |
|   1001 | zhangsan |      99 |        999 |
|   1002 | 王五     |      99 |        999 |
+--------+----------+---------+------------+
3 rows in set, 6 warnings (0.00 sec)

MySQL 字串拼接詳解