mysql 查询字段拼接

Mysql的查询结果行字段拼接，可以用下面两个函数实现：

1. concat函数

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<pre style="margin: 0px; padding: 0px; white-space: pre-wrap; overflow-wrap: break-word; font-family: "Courier New" !important; font-size: 12px !important;">mysql> select concat('1','2','3') from test ; +---------------------+
| concat('1','2','3') |
+---------------------+
| 123 |
+---------------------+</pre>

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如果连接串中存在NULL，则返回结果为NULL：

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<pre style="margin: 0px; padding: 0px; white-space: pre-wrap; overflow-wrap: break-word; font-family: "Courier New" !important; font-size: 12px !important;">mysql> select concat('1','2',NULL,'3') from test ; +--------------------------+
| concat('1','2',NULL,'3') |
+--------------------------+
| NULL |
+--------------------------+</pre>

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2. concat_ws函数

concat(separator,str1,str2,...) 代表 concat with separator ，是concat()的特殊形式。第一个参数是其它参数的分隔符。分隔符的位置放在要连接的两个字符串之间。分隔符可以是一个字符串，也可以是其它参数。

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<pre style="margin: 0px; padding: 0px; white-space: pre-wrap; overflow-wrap: break-word; font-family: "Courier New" !important; font-size: 12px !important;">mysql> select concat_ws(':','1','2','3') from test ; +----------------------------+
| concat_ws(':','1','2','3') |
+----------------------------+
| 1:2:3 |
+----------------------------+</pre>

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分隔符为NULL，则返回结果为NULL：

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<pre style="margin: 0px; padding: 0px; white-space: pre-wrap; overflow-wrap: break-word; font-family: "Courier New" !important; font-size: 12px !important;">mysql> select concat_ws(NULL,'1','2','3') from test; +-----------------------------+
| concat_ws(NULL,'1','2','3') |
+-----------------------------+
| NULL |
+-----------------------------+</pre>

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如果参数中存在NULL，则会被忽略：

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<pre style="margin: 0px; padding: 0px; white-space: pre-wrap; overflow-wrap: break-word; font-family: "Courier New" !important; font-size: 12px !important;">mysql> select concat_ws(':','1','2',NULL,NULL,NULL,'3') from test ; +-------------------------------------------+
| concat_ws(':','1','2',NULL,NULL,NULL,'3') |
+-------------------------------------------+
| 1:2:3 |
+-------------------------------------------+</pre>

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可以对NULL进行判断，并用其它值进行替

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<pre style="margin: 0px; padding: 0px; white-space: pre-wrap; overflow-wrap: break-word; font-family: "Courier New" !important; font-size: 12px !important;">mysql> select concat_ws(':','1','2',ifNULL(NULL,'0'),'3') from bank limit 1; +---------------------------------------------+
| concat_ws(':','1','2',ifNULL(NULL,'0'),'3') |
+---------------------------------------------+
| 1:2:0:3 |
+---------------------------------------------+</pre>

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