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函式有一個特點就是有一個值為NULL那麼不管第二個字元有多少內容都返回為空了，所以我們使用IFNULL來取出想取出的資料。 如果我們要對取出的資料進行處理，如需要將男生定義為1，女生定義為2，我們可以使用case when來處理資料：

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'
 
;

這裡我們使用LIKE來判斷是男生還是女生，因為有的資料庫存的是“男”“女”，有的資料庫存的是“男生”“女生”等。 如果我們取出的名字欄位有轉義字元，那麼需要我們處理轉義字元：

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';

在這裡我們使用replace對名字中的轉義字元進行處理。 如果取出的資料有重複，且我們判定重複的條件為：幾個欄位相同才判定為相同，而distinct只能處理一個欄位重複的情況，這時我們使用group by來進行處理：

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';

這裡我們通過姓名和電話號碼兩個欄位判定是否是相同資料。