mysql判断是否登录语句

-- 单表查询示例
SELECT * FROM users WHERE username = 'exampleUser' AND password = 'hashedPassword';

-- 多表关联查询示例
SELECT u.*, r.role_name
FROM users u
JOIN user_roles ur ON u.user_id = ur.user_id
JOIN roles r ON ur.role_id = r.role_id
WHERE u.username = 'exampleUser' AND u.password = 'hashedPassword';

-- 存储密码时进行哈希处理
INSERT INTO users (username, password) VALUES ('exampleUser', SHA2('plainTextPassword', 256));

-- 使用预处理语句
PREPARE stmt FROM 'SELECT * FROM users WHERE username = ? AND password = ?';
SET @username = 'exampleUser';
SET @password = SHA2('plainTextPassword', 256);
EXECUTE stmt USING @username, @password;
DEALLOCATE PREPARE stmt;

-- 添加索引
CREATE INDEX idx_username ON users(username);