I want to find max lenght of string from a tabe in SQLite

Question

SELECT MAX(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`) 
FROM `student_info`

by using similar type of query i want to find one max value of string so i can manage table properly.

what is the proper query for SQLite?

Answer 1

There is no need for UNION.

Get each column's max length with MAX() aggregate function and then use MAX() scalar function to get the greatest of all:

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

Or:

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

See a simplified demo.