MySQL 语句级避免重复插入—— Insert Select Not Exist

想要插入一条数据，要避免重复插入，又不想折腾两回数据库连接操作，可以参考如下办法

INSERT INTO table(column1,column2,column3 ...columnN)
SELECT value1,value2,value3 ...valueN
FROM dual
WHERE NOT EXISTS(
      SELECT *
      FROM table
      WHERE value = ?

);

INSERT INTO user(username,password) SELECT 123,123 FROM DUAL WHERE NOT EXISTS(SELECT username FROM user WHERE username='123');