关于去重插入

旨在减少不必要的麻烦，避免因程序多次调用接口产生重复数据和开销。

INSERT into user_ceshi(username,password) SELECT '张三','123456' FROM DUAL WHERE NOT EXISTS
(SELECT username,password FROM user_ceshi WHERE username='张三' AND password ='123456')

js中用变量替换的方法，这里用的是express

方法一：模板字符串替换变量

function insert(username.password) {
  return new Promise((resolve, reject) => {
	  let sql= `INSERT into user_ceshi(username,password) SELECT ${username},${password} FROM DUAL WHERE not EXISTS(SELECT username,password FROM user_ceshi WHERE username=${username} AND password =${password})`
    mysqldb.query(
     sql,
      (err, result) => {
        if (err) {
          reject(err);
        } else {
          resolve(result);
        }
      }
    );
  });
}

方法二：使用？

function insert(username,password) {
  return new Promise((resolve, reject) => {
	  let sql="INSERT into user_ceshi(username,password)  
SELECT ?,? FROM DUAL WHERE not EXISTS
(SELECT username,password FROM user_ceshi WHERE username=? AND password =?)"  mysqldb.query(
     sql,
      [
        username,
        password,
        username,
        password
      ],
      (err, result) => {
        if (err) {
          reject(err);
        } else {
          resolve(result);
        }
      }
    );
  });
}