What is N in nvarchar in SQL Server

Prepared Statement get wrong result in MYSQL

Zimmerman

I have a table with design

CREATE TABLE IF NOT EXISTS InsuranceContract (
`id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
`enquiryCode` VARCHAR(20) DEFAULT NULL,
`contractCode` VARCHAR(20) DEFAULT NULL,
`createdAt` DATETIME DEFAULT CURRENT_TIMESTAMP (),
`updatedAt` DATETIME DEFAULT CURRENT_TIMESTAMP () ON UPDATE CURRENT_TIMESTAMP (),
UNIQUE KEY (`enquiryCode`)) ENGINE=INNODB DEFAULT CHARSET=UTF8 COLLATE = UTF8_BIN;

Then I was created a procedure like this

DROP procedure IF EXISTS `sp_insurance_contract_get`;
DELIMITER $$ 
CREATE PROCEDURE `sp_insurance_contract_get` (enquiryCode VARCHAR(20), contractCode VARCHAR(20)) 
BEGIN
    SET @t1 = "SELECT * FROM InsuranceContract 
               WHERE InsuranceContract.enquiryCode = enquiryCode 
                     AND InsuranceContract.contractCode = contractCode;";
    PREPARE param_stmt FROM @t1;
  EXECUTE param_stmt;
  DEALLOCATE PREPARE param_stmt;
END$$ 
DELIMITER ;

And I was executed this procedure in MySQL Workbench by this command:

CALL sp_insurance_contract_get('EQ000000000014', '3001002');

I expected I will receive 1 row result but it selected all records in this table. If I copy and create exactly this @t1 into plain SQL not using statement, it's correct.

Please help me to fix this error. I'm using MySQL 8.0.19