MySQL rows concatenation to fetch maximum corresponding value from duplicate IDs?

For this, you can use GROUP BY clause. To find maximum value, use MAX() function. Let us first create a table −

mysql> create table DemoTable1804
     (
     Id int,
     Marks1 int,
     Marks2 int,
     Marks3 int
     );
Query OK, 0 rows affected (0.00 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable1804 values(1,56,89,34);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1804 values(1,98,null,94);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1804 values(2,34,45,78);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1804 values(2,null,67,null);
Query OK, 1 row affected (0.00 sec)

Display all records from the table using select statement −

mysql> select * from DemoTable1804;

This will produce the following output −

+------+--------+--------+--------+
| Id   | Marks1 | Marks2 | Marks3 |
+------+--------+--------+--------+
|    1 |     56 |    89 |      34 |
|    1 |     98 |  NULL |      94 |
|    2 |     34 |    45 |      78 |
|    2 |    NULL|    67 |    NULL |
+------+--------+--------+--------+
4 rows in set (0.00 sec)

Here is the query for rows concatenation and finding maximum value −

mysql> select Id,max(Marks1),max(Marks2),max(Marks3)
     from DemoTable1804
     group by Id;

This will produce the following output −

+------+-------------+-------------+-------------+
| Id   | max(Marks1) | max(Marks2) | max(Marks3) |
+------+-------------+-------------+-------------+
|    1 |          98 |          89 |          94 |
|    2 |          34 |          67 |          78 |
+------+-------------+-------------+-------------+
2 rows in set (0.00 sec)