如何从mysql数据库中查询指定字段且符合条件的数据 ，然后拼接成json字符串，最后导出json文件

SELECT CONCAT("{'name':'",IFNULL(Name,''),"',","'sex':'",IFNULL(Sex,''),"',","'age':'",IFNULL(Age,''),"',",
"'phone':'",IFNULL(Phone,''),"',","'birthday':'",IFNULL(Birthday,''),"'","},") from Student  where Status=9
 INTO OUTFILE '/var/lib/mysql-files/query_student.json';

在MySQL中CONCAT函数有一个特点就是有一个值为NULL那么不管第二个字符有多少内容都返回为空了，所以我们使用IFNULL来取出想取出的数据。
如果我们要对取出的数据进行处理，如需要将男生定义为1，女生定义为2，我们可以使用case when来处理数据：

SELECT CONCAT("{'name':'",IFNULL(Name,''),"',","'sex':'",IFNULL(case when Sex LIKE "%男%" then 1 when Sex LIKE "%女%" then 2 end,''),"',","'age':'",IFNULL(Age,''),"',",
"'phone':'",IFNULL(Phone,''),"',","'birthday':'",IFNULL(Birthday,''),"'","},") from Student  where Status=9
 INTO OUTFILE '/var/lib/mysql-files/query_student.json';

这里我们使用LIKE来判断是男生还是女生，因为有的数据库存的是“男”“女”，有的数据库存的是“男生”“女生”等。
如果我们取出的名字字段有转义字符，那么需要我们处理转义字符：

SELECT CONCAT("{'name':'",IFNULL(REPLACE(Name,'\'','\\\''),''),"',","'sex':'",IFNULL(case when Sex LIKE "%男%" then 1 when Sex LIKE "%女%" then 2 end,''),"',","'age':'",IFNULL(Age,''),"',",
"'phone':'",IFNULL(Phone,''),"',","'birthday':'",IFNULL(Birthday,''),"'","},") from Student  where Status=9
 INTO OUTFILE '/var/lib/mysql-files/query_student.json';

在这里我们使用replace对名字中的转义字符进行处理。
如果取出的数据有重复，且我们判定重复的条件为：几个字段相同才判定为相同，而distinct只能处理一个字段重复的情况，这时我们使用group by来进行处理：

SELECT CONCAT("{'name':'",IFNULL(REPLACE(Name,'\'','\\\''),''),"',","'sex':'",IFNULL(case when Sex LIKE "%男%" then 1 when Sex LIKE "%女%" then 2 end,''),"',","'age':'",IFNULL(Age,''),"',",
"'phone':'",IFNULL(Phone,''),"',","'birthday':'",IFNULL(Birthday,''),"'","},") from Student  where Status=9 GROUP BY name,phone
 INTO OUTFILE '/var/lib/mysql-files/query_student.json';

这里我们通过姓名和电话号码两个字段判定是否是相同数据。