The different forms of COUNT( ) can be very useful for counting missing values : COUNT « Aggregate Functions « SQL / MySQL






  1. SQL / MySQL
  2. Aggregate Functions
  3. COUNT

The different forms of COUNT( ) can be very useful for counting missing values

        
mysql>
mysql>
mysql> CREATE TABLE expt
    -> (
    ->  subject VARCHAR(10),
    ->  test    VARCHAR(5),
    ->  score   INT
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Jane','A',47);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Jane','B',50);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Jane','C',NULL);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Jane','D',NULL);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Marvin','A',52);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Marvin','B',45);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Marvin','C',53);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO expt (subject,test,score) VALUES('Marvin','D',NULL);
Query OK, 1 row affected (0.00 sec)

mysql>
mysql> SELECT * FROM expt;
+---------+------+-------+
| subject | test | score |
+---------+------+-------+
| Jane    | A    |    47 |
| Jane    | B    |    50 |
| Jane    | C    |  NULL |
| Jane    | D    |  NULL |
| Marvin  | A    |    52 |
| Marvin  | B    |    45 |
| Marvin  | C    |    53 |
| Marvin  | D    |  NULL |
+---------+------+-------+
8 rows in set (0.00 sec)

mysql>
mysql> SELECT COUNT(*) - COUNT(score) AS missing FROM expt;
+---------+
| missing |
+---------+
|       3 |
+---------+
1 row in set (0.00 sec)

mysql> drop table expt;
Query OK, 0 rows affected (0.00 sec)

mysql>








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