¿Consulta MySQL para seleccionar un recuento en dos condiciones separadas?

Utilice la declaración CASE para esto. Primero creemos una tabla:

mysql> create table DemoTable
   -> (
   -> StudentMarks int,
   -> isValid tinyint(1)
   -> );
Query OK, 0 rows affected (0.68 sec)

Inserte algunos registros en la tabla usando el comando insertar:

mysql> insert into DemoTable values(45,0);
Query OK, 1 row affected (0.15 sec)

mysql> insert into DemoTable values(78,1);
Query OK, 1 row affected (0.26 sec)

mysql> insert into DemoTable values(45,1);
Query OK, 1 row affected (0.31 sec)

mysql> insert into DemoTable values(78,1);
Query OK, 1 row affected (0.22 sec)

mysql> insert into DemoTable values(45,0);
Query OK, 1 row affected (0.17 sec)

mysql> insert into DemoTable values(82,1);
Query OK, 1 row affected (0.12 sec)

mysql> insert into DemoTable values(62,1);
Query OK, 1 row affected (0.14 sec)

Muestre todos los registros de la tabla usando la declaración de selección:

mysql> select *from DemoTable;

Producción

+--------------+---------+
| StudentMarks | isValid |
+--------------+---------+
| 45           | 0       |
| 78           | 1       |
| 45           | 1       |
| 78           | 1       |
| 45           | 0       |
| 82           | 1       |
| 62           | 1       |
+--------------+---------+
7 rows in set (0.00 sec)

Aquí está la consulta para seleccionar un recuento en dos condiciones distintas:

mysql> select StudentMarks,
   -> sum(case
   -> when StudentMarks=45
   -> then case when isValid = 1 then 1 else 0 end
   -> else 1 end
   -> ) AS Freq
   -> from DemoTable
   -> group by StudentMarks;

Producción

+--------------+------+
| StudentMarks | Freq |
+--------------+------+
| 45           | 1    |
| 78           | 2    |
| 82           | 1    |
| 62           | 1    |
+--------------+------+
4 rows in set (0.00 sec)