Mysql, check if all rows of each column have the same value - mysql

Question

A friend of mine wants to check if all the rows, of each of the columns of a table, have the same value.
If they do, then print the value.
Else just print an empty string or null or something.
Imagine this table for example:
+--------+----------+-----+
| Name | Lastname | Age |
+--------+----------+-----+
| Peter | White | 30 |
| Marry | Jane | 30 |
| John | Doe | 30 |
+--------+----------+-----+
The result of the wanted query would output the following:
+--------+----------+-----+
| Name | Lastname | Age |
+--------+----------+-----+
| NULL | NULL | 30 |
+--------+----------+-----+
I tried to create a function where I would get the columns of a given table, loop through each column name and execute a query. But since I am not familiar with Mysql I obviously miss something out and I can't figure out how to achieve what I'm trying to do here.
DROP PROCEDURE IF EXISTS test;
DELIMITER //
create procedure test()
begin
declare i int(11);
declare col_name varchar(50);
declare num_rows int(11);
DECLARE col_names CURSOR FOR
SELECT column_name
FROM information_schema.columns
WHERE table_name='name_of_my_table' and table_schema='name_of_db';
select FOUND_ROWS() as num_rows;
set i = 1;
open col_names;
the_loop: LOOP
IF i > num_rows THEN
CLOSE col_names;
LEAVE the_loop;
END IF;
FETCH col_names
INTO col_name;
-- Here I would like to perform a query for each column
select count(*), col_name from name_of_my_table group by col_name;
-- Then I was thinking of making an if/ else condition to check
-- if I get more than 1 result per column, implying that
-- not all rows have the same value for this column.
SET i = i + 1;
END LOOP the_loop;
CLOSE col_names;
END//
DELIMITER ;
call test;
What this outputs is the count and the column name of the last column found, which does make sense.
I am not sure if what I am trying to do is possible with Mysql only, I can easily do that in PHP but I am wondering if I can do that with a single query as well.

Answer 1

Try the below query.
select if(count(distinct(Name))=1,Name,null), if(count(distinct(Lastname))=1,Lastname,null), if(count(distinct(Age))=1,Age,null)
from your_table;