您的查询大部分是正确的，只是做了如下的小改动

SELECT MAX(count) from (
  SELECT MAX(LENGTH(`name`)) count FROM `student_info` 
UNION SELECT MAX(LENGTH(`address`)) count FROM `student_info` 
UNION SELECT MAX(LENGTH(`class`)) count FROM `student_info` 
UNION SELECT MAX(LENGTH(`roll`)) count FROM `student_info` 
UNION SELECT MAX(LENGTH(`subject`)) count FROM `student_info`
) ;

Try Here

您可以使用联合限制方法：

SELECT name, field, field_length
FROM
(
    SELECT 'name' AS name, name AS field, LENGTH(name) AS field_length FROM student_info
    UNION ALL
    SELECT 'address', address, LENGTH(address) FROM student_info
    UNION ALL
    SELECT 'class', class, LENGTH(class) FROM student_info
    UNION ALL
    SELECT 'role', role, LENGTH(role) FROM student_info
    UNION ALL
    SELECT 'subject', subject, LENGTH(subject) FROM student_info
) t
ORDER BY field_length DESC
LIMIT 1;

上面的方法是形成一个包含两列的中间表，其中一列包含感兴趣的5列中的所有字符串，另一列包含它们的长度。然后我们使用限制查询来查找最大长度条目和列名。

不需要UNION。
使用MAX() aggregate function获取每列的最大长度，然后使用MAX（）标量函数获取最大值：

SELECT MAX(
         MAX(LENGTH(name)),
         MAX(LENGTH(address)), 
         MAX(LENGTH(class)), 
         MAX(LENGTH(roll)), 
         MAX(LENGTH(subject))
       ) AS max_count 
FROM student_info;