concat介绍
CONCAT(字串1, 字串2, 字串3, ...): 将字串1、字串2、字串3，等字串连在一起。
示例
SELECT CONCAT(region_name,store_name) FROM Geography
WHERE store_name = 'Boston';
应用场景
模糊查询
concat:
Used to connect the field, or the characters. But as long as the field is connected with a value of null, then the concat get is null.
用于连接字段或者字符，如果用于连接的字符中有一个值为null，则结果为null.
This is a very dangerous thing. I believe the number of fields connected with the concat the results people want most is not the case:
大家使用concat的意图是：连接两个字符A,B，如果B为null，则连接结果为A
concat (A, B), if B is null, then concat (A, B) = null
What we want is this:
就像这样：
Connection A, B, if B is null, then the connection should be A.
解决这个问题，可以使用CONCAT_WS()
At this time should CONCAT_WS (separator, str1, str2 ,...)
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