Mysql 函数concat、concat_ws和group_concat
本文介绍的是MySQL中3个函数的使用，主要是针对字符串的连接合并处理：
concat
concat_ws
group_concat
concat
concat()函数是将多个字符串组合在一起，形成一个大的字符串；如果连接的字符串中存在一个为NULL，则输出的结果为NULL，语法格式为：
concat(str1,str2,....strn)
3个例子🌰说明具体使用，以下面这个表中的第一条记录为例：
-- 1、字符之间不加连接符
mysql> select concat("01","赵雷","男");
+-----------------------------+
| concat("01","赵雷","男") |
+-----------------------------+
| 01赵雷男 |
+-----------------------------+
1 row in set (0.00 sec)
-- 2、字符之间添加连接符
mysql> select concat("01-","赵雷-","男");
+-------------------------------+
| concat("01-","赵雷-","男") |
+-------------------------------+
| 01-赵雷-男 |
+-------------------------------+
1 row in set (0.00 sec)
-- 3、忽略空字符串
mysql> mysql> select concat("01","赵雷","","男");
+--------------------------------+
| concat("01","赵雷","","男") |
+--------------------------------+
| 01赵雷男 |
+--------------------------------+
1 row in set (0.00 sec)
-- 4、存在NULL的情况
mysql> select concat("01","赵雷",NULL,"男"); -- 结果直接显示为NULL
+----------------------------------+
| concat("01","赵雷",NULL,"男") |
+----------------------------------+
| NULL |
+----------------------------------+
1 row in set (0.01 sec)
注意两种情况的不同：
concat_ws
concat_ws()函数相比较于concat()多了一个指定的连接符号，语法为：
concat_ws(separator, str1, str2, str3)
- 第一个参数是连接的符号
- 后面的参数是待连接的字符
连接符要放在待连接的字符之间；分隔符也可以是一个字符串，也可以是其他的参数，需要注意的是：
- 如果分隔符是NULL，结果为NULL
- 函数后忽略任何分割符参数后的NULL值（分隔符之后的NULL值）：连接的时候跳过NULL值
- concat_ws不会忽略空字符串；concat会忽略空字符串
下面通过几个例子来说明使用方法：
-- 1、指定不同的连接符号：分别指定逗号和加号
mysql> select concat_ws(",","01","赵雷","男");
+------------------------------------+
| concat_ws(",","01","赵雷","男") |
+------------------------------------+
| 01,赵雷,男 |
+------------------------------------+
1 row in set (0.00 sec)
mysql> select concat_ws("+","01","赵雷","男");
+------------------------------------+
| concat_ws("+","01","赵雷","男") |
+------------------------------------+
| 01+赵雷+男 |
+------------------------------------+
1 row in set (0.00 sec)
-- 2、不忽略空字符串
mysql> select concat_ws("+","01","赵雷","","男");
+---------------------------------------+
| concat_ws("+","01","赵雷","","男") |
+---------------------------------------+
| 01+赵雷++男 |
+---------------------------------------+
1 row in set (0.00 sec)
-- 3、忽略NULL；不管几个NULL都会忽略
mysql> select concat_ws("+","01","赵雷",NULL,"男");
+-----------------------------------------+
| concat_ws("+","01","赵雷",NULL,"男") |
+-----------------------------------------+
| 01+赵雷+男 |
+-----------------------------------------+
1 row in set (0.00 sec)
-- 忽略两个NULL
mysql> select concat_ws("+","01",NULL,"赵雷",NULL,"男");
+----------------------------------------------+
| concat_ws("+","01",NULL,"赵雷",NULL,"男") |
+----------------------------------------------+
| 01+赵雷+男 |
+----------------------------------------------+
1 row in set (0.00 sec)
group_concat
group：分组的意思；concat：连接。合起来就是分组连接，具体语法为：
<pre spellcheck="false" class="md-fences md-end-block md-fences-with-lineno ty-contain-cm modeLoaded" lang="sql" cid="n84" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 0px; width: inherit; background-position: initial initial; background-repeat: initial initial;"> GROUP_CONCAT(DISTINCT expression ORDER BY expression SEPARATOR sep);</pre>
<pre mdtype="fences" cid="n118" lang="sql" class="md-fences md-end-block md-fences-with-lineno ty-contain-cm modeLoaded" spellcheck="false" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background-color: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 0px; width: inherit; background-position: initial initial; background-repeat: initial initial;"> set session group_concat_max_len=18783847439738273; -- 防止超出范围数据被截掉</pre>
下面通过这张成绩表Score来讲解：
DISTINCT子句用于在连接分组之前消除组中的重复值ORDER BY连接之前按升序或者降序排列。默认是升序SEPARATOR指定在组中的值之间插入的文字值。如果不指定分隔符，则GROUP_CONCAT函数使用逗号(，)作为默认分隔符函数会自动忽略NULL值，如果所有的参数都是NULL，则结果返回NULL
GROUP_CONCAT函数返回二进制或非二进制字符串，取决于参数。 默认情况下，返回字符串的最大长度为1024。通过在SESSION或GLOBAL级别设置group_concat_max_len系统变量来扩展最大长度。
-- 1、将每个学生的成绩单独列出来
mysql> select s_id, group_concat(s_score) from Score group by s_id;
+------+-----------------------+
| s_id | group_concat(s_score) |
+------+-----------------------+
| 01 | 80,90,96 |
| 02 | 70,60,80 |
| 03 | 80,81,85 |
| 04 | 50,40,30 |
| 05 | 76,87 |
| 06 | 43,56 |
| 07 | 89,94 |
+------+-----------------------+
7 rows in set (0.01 sec)
-- 2、指定连接符+
mysql> select s_id, group_concat(s_score separator "+") from Score group by s_id;
+------+-------------------------------------+
| s_id | group_concat(s_score separator "+") |
+------+-------------------------------------+
| 01 | 80+90+96 |
| 02 | 70+60+80 |
| 03 | 80+81+85 |
| 04 | 50+40+30 |
| 05 | 76+87 |
| 06 | 43+56 |
| 07 | 89+94 |
+------+-------------------------------------+
7 rows in set (0.00 sec)
-- 3、指定排序的字段
-- 分数s_score已经完成了排序（指定了降序）；上面的结果不指定则默认是降序
mysql> select s_id, group_concat(distinct s_score order by s_score desc separator "+") from Score group by s_id;
+------+--------------------------------------------------------------------+
| s_id | group_concat(distinct s_score order by s_score desc separator "+") |
+------+--------------------------------------------------------------------+
| 01 | 96+90+80 |
| 02 | 80+70+60 |
| 03 | 85+81+80 |
| 04 | 50+40+30 |
| 05 | 87+76 |
| 06 | 56+43 |
| 07 | 94+89 |
+------+--------------------------------------------------------------------+
7 rows in set (0.00 sec)
-- 4、去重操作
-- distinct s_score表示对分数去重，取出每个学生的不同分数（表中每个学生的分数都不相同，结果同上）
mysql> select s_id, group_concat(distinct s_score order by s_score desc separator "+") from Score group by s_id;
+------+--------------------------------------------------------------------+
| s_id | group_concat(distinct s_score order by s_score desc separator "+") |
+------+--------------------------------------------------------------------+
| 01 | 96+90+80 |
| 02 | 80+70+60 |
| 03 | 85+81+80 |
| 04 | 50+40+30 |
| 05 | 87+76 |
| 06 | 56+43 |
| 07 | 94+89 |
+------+--------------------------------------------------------------------+
7 rows in set (0.00 sec)
distinct 和order by 后面的字段是相同的
