无法找到字符串文字操作员; conteudo;使用＆＃x27; const char [43]＆＃x27;，＆＃x27; unsigned int＆＃x27;争论

Question

我遇到了问题，在变量上向MySQL数据库发送数据。问题是我试图通过变量发送RFID数据。

String INSERT_SQL ="INSERT INTO database.table VALUES('0 ',"conteudo")";

它给出了这个错误。

无法找到字符串字符串运算符'Operator“ operator”“ conteudo”，带有'const char [43]'，'unsigned int'参数，

这是完整的代码。

#include <WiFi.h>
#include <SPI.h>
#include <MFRC522.h>
#include <Servo.h>
#include <MySQL_Generic.h>    

Servo servo;

#define SS_PIN 21
#define RST_PIN 22

MFRC522 mfrc522(SS_PIN, RST_PIN);



const char* ssid = "";
const char* password = "";

char user[] = "";
char passw[] = "";

String conteudo = "";


#define MYSQL_DEBUG_PORT      Serial

#define MYSQL_LOGLEVEL      0

#define USING_HOST_NAME     true

#if USING_HOST_NAME

char server[] = ""; 
char default_database[] = "";
char default_table[]    = "";

#else

  IPAddress server(192, 168, 0, 250);
#endif

uint16_t server_port = 3306;


MySQL_Connection conn((Client *)&client);
MySQL_Query *query_mem;

int led_CH1 = 14;
int pos = 0;

void setup()
{
  pinMode(led_CH1, OUTPUT);

  Serial.begin(115200);

  while (!Serial && millis() < 5000);

  SPI.begin();

  iniciarWiFi();

  MYSQL_DISPLAY1("Connecting to", ssid);
  MYSQL_DISPLAY3("Connecting to SQL Server @", server, ", Port =", server_port);
  MYSQL_DISPLAY5("User =", user, ", PW =", passw, ", DB =", default_database);

  mfrc522.PCD_Init();

   servo.attach(25);
   servo.write(100);

  Serial.println("Aproxime o seu cartao do leitor...");
  Serial.println();
}

void loop()
{

  
  if ( ! mfrc522.PICC_IsNewCardPresent())
  {
    return;
  }
  if ( ! mfrc522.PICC_ReadCardSerial())
  {
    return;
  }
  
  Serial.print("UID da tag :");
  
  byte letra;
  for (byte i = 0; i < mfrc522.uid.size; i++)
  {
    Serial.print(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " ");
    Serial.print(mfrc522.uid.uidByte[i], HEX);
    conteudo.concat(String(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " "));
    conteudo.concat(String(mfrc522.uid.uidByte[i], HEX));
  }
  Serial.println();
  Serial.print("Mensagem : ");
  conteudo.toUpperCase();

  if (conteudo.substring(1) == "B0 21 A4 39")
  {
    digitalWrite(led_CH1, HIGH);
    Serial.println("Cartao1 - Acesso concedido!");
    Serial.println();

    String INSERT_SQL ="INSERT INTO database.table VALUES('0 ',"conteudo")";

     if (conn.connectNonBlocking(server, server_port, user, passw) != RESULT_FAIL)
  {
    delay(500);
    runInsert();
    conn.close();                     // close the connection
  }
   else 
  {
    MYSQL_DISPLAY("\nConnect failed. Trying again on next iteration.");
  }

  delay(100);
  
    delay(3000);
    for (pos = 0; pos <= 180; pos += 1) { // goes from 0 degrees to 180 degrees
    // in steps of 1 degree
    servo.write(pos);              // tell servo to go to position in variable 'pos'
    delay(15);                       // waits 15ms for the servo to reach the position
    }
    delay(1500);
  
     for (pos = 180; pos >= 0; pos -= 1) { // goes from 180 degrees to 0 degrees
    servo.write(pos);              // tell servo to go to position in variable 'pos'
    delay(15);                       // waits 15ms for the servo to reach the position
  }
    digitalWrite(led_CH1, LOW);
    
  }

  if (conteudo.substring(1) != "B0 21 A4 39")
  {
    Serial.println("Cartao2 - Acesso negado !!");
    Serial.println();
  }
  delay(1000);
}
void iniciarWiFi() {
  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  Serial.print("A conectar ao WiFi .");
  while (WiFi.status() != WL_CONNECTED)
  {
    Serial.print('.');
    delay(1000);
  }
  Serial.println(WiFi.localIP());
}

void runInsert()
{
  // Initiate the query class instance
  MySQL_Query query_mem = MySQL_Query(&conn);

  if (conn.connected())
  {
    MYSQL_DISPLAY(INSERT_SQL);
    
    // Execute the query
    // KH, check if valid before fetching
    if ( !query_mem.execute(INSERT_SQL.c_str()) )
    {
      MYSQL_DISPLAY("Insert error");
    }
    else
    {
      MYSQL_DISPLAY("Data Inserted.");
    }
  }
  else
  {
    MYSQL_DISPLAY("Disconnected from Server. Can't insert.");
  }
}

Answer 1

您正在尝试将字符串嵌入另一个字符串中。这是不允许的。

                                             This " will mark end of string
-----------------------------------------------------------V---- 
String INSERT_SQL ="INSERT INTO database.table VALUES('0 '," conteudo")";

如果要在字符串文字中使用“”，则需要这样逃脱：

String INSERT_SQL ="INSERT INTO database.table VALUES('0 ',\"conteudo\")";

如果要将conteudo的值插入SQL查询中，则需要执行下列：

String INSERT_SQL ="INSERT INTO database.table VALUES('0 ', " + conteudo + ")";