使用nodejs检查mysql数据库中是否存在电子邮件

当用户输入表单时需要检查电子邮件是否存在于 mysql 中.从表单 requser.email 将用户通过电子邮件输入表单.如何将 requser.email 与数据库进行比较以找到匹配项.

Need to check email is exists or not in mysql when user enters in the form. From form requser.email will have user emaild entered in form. How to compare requser.email with database to find match.

 router.post('/signup', function(req, res, next) {
    var requser = req.body.data.user;
  var hash = bcrypt.hashSync(req.body.data.user.password)
  var str = "";

  /*email check start*/

  /*email check end*/
  switch (requser.role) {
    case "approver":
      str = 'INSERT INTO users (name, email, password,role) values("' + requser.name + '","' + requser.email + '","' + hash + '","approver")';
      break;
    case "user":
      str = 'INSERT INTO users (name, email, password,role) values("' + requser.name + '","' + requser.email + '","' + hash + '","user")';
      break;
    case "admin":
      str = 'INSERT INTO users (name, email, password,role) values("' + requser.name + '","' + requser.email + '","' + hash + '","admin")';
      break;
  }
  if (!str) {
    return res.json({
      "status": "error",
      "data": {
        "redirect": "/"
      }
    });
  }
  connection.query(str, function(error, rows) {
    if (error) {
      console.log("error ocurred", error);
    }
    //console.log(rows);
    res.json({
      "status": error ? "error" : "success",
      "data": rows || ""
    })
  });
});