Use COUNT, GROUP and HAVING

/*

mysql> select * from Item;
+----+-----------+---------+---------+----------+------------+----------+---------------------+
| ID | Name      | InStock | OnOrder | Reserved | Department | Category | RowUpdate           |
+----+-----------+---------+---------+----------+------------+----------+---------------------+
|  1 | Bloodshot |      10 |       5 |        1 | Popular    | Rock     | 2005-10-09 09:19:45 |
|  2 | Most      |      10 |       5 |        2 | Classical  | Opera    | 2005-10-09 09:19:45 |
|  3 | Jazz      |      17 |       4 |        3 | Popular    | Jazz     | 2005-10-09 09:19:45 |
|  4 | Class     |       9 |       4 |        4 | Classical  | Dance    | 2005-10-09 09:19:45 |
|  5 | Violin    |      24 |       2 |        5 | Classical  | General  | 2005-10-09 09:19:45 |
|  6 | Cha Cha   |      16 |       6 |        6 | Classical  | Vocal    | 2005-10-09 09:19:45 |
|  7 | Blues     |       2 |      25 |        7 | Popular    | Blues    | 2005-10-09 09:19:45 |
|  8 | Pure      |      32 |       3 |       18 | Popular    | Jazz     | 2005-10-09 09:19:45 |
|  9 | Mud       |      12 |      15 |       19 | Popular    | Country  | 2005-10-09 09:19:45 |
| 10 | The       |       5 |      20 |       11 | Popular    | New Age  | 2005-10-09 09:19:45 |
| 11 | Embrace   |      24 |      11 |       12 | Popular    | New Age  | 2005-10-09 09:19:45 |
| 12 | Magic     |      42 |      17 |       13 | Classical  | General  | 2005-10-09 09:19:45 |
| 13 | Lake      |      25 |      44 |       24 | Classical  | Dance    | 2005-10-09 09:19:45 |
| 14 | LaLala    |      20 |      10 |        5 | Classical  | Opera    | 2005-10-09 09:19:45 |
| 15 | Soul      |      15 |      30 |       16 | Popular    | Blues    | 2005-10-09 09:19:45 |
| 16 | Stages    |      42 |       0 |        7 | Popular    | Blues    | 2005-10-09 09:19:45 |
| 17 | Six       |      16 |       8 |        6 | Classical  | General  | 2005-10-09 09:19:45 |
+----+-----------+---------+---------+----------+------------+----------+---------------------+
17 rows in set (0.00 sec)

mysql> SELECT Category, COUNT(*) AS Total
    -> FROM Item
    -> WHERE Department='Popular'
    -> GROUP BY Category
    -> HAVING Total < 3;
+----------+-------+
| Category | Total |
+----------+-------+
| Country  |     1 |
| Jazz     |     2 |
| New Age  |     2 |
| Rock     |     1 |
+----------+-------+
4 rows in set (0.00 sec)

*/

Drop table Item;

CREATE TABLE Item
(
   ID SMALLINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
   Name VARCHAR(50) NOT NULL,
   InStock SMALLINT UNSIGNED NOT NULL,
   OnOrder SMALLINT UNSIGNED NOT NULL,
   Reserved SMALLINT UNSIGNED NOT NULL,
   Department ENUM('Classical', 'Popular') NOT NULL,
   Category VARCHAR(20) NOT NULL,
   RowUpdate TIMESTAMP NOT NULL
);


INSERT INTO Item (Name, InStock, OnOrder, Reserved, Department, Category)
          VALUES ('Bloodshot',      10,      5,       1, 'Popular',      'Rock'),
                 ('Most',           10,      5,       2, 'Classical',    'Opera'),
                 ('Jazz',           17,      4,       3, 'Popular',      'Jazz'),
                 ('Class',           9,      4,       4, 'Classical',    'Dance'),
                 ('Violin',         24,      2,       5, 'Classical',    'General'),
                 ('Cha Cha',        16,      6,       6, 'Classical',    'Vocal'),
                 ('Blues',           2,     25,       7, 'Popular',      'Blues'),
                 ('Pure',           32,      3,      18, 'Popular',      'Jazz'),
                 ('Mud',            12,     15,      19, 'Popular',      'Country'),
                 ('The',             5,     20,      11, 'Popular',      'New Age'),
                 ('Embrace',        24,     11,      12, 'Popular',      'New Age'),
                 ('Magic',          42,     17,      13, 'Classical',    'General'),
                 ('Lake',           25,     44,      24, 'Classical',    'Dance'),
                 ('LaLala',         20,     10,       5, 'Classical',    'Opera'),
                 ('Soul',           15,     30,      16, 'Popular',      'Blues'),
                 ('Stages',         42,      0,       7, 'Popular',      'Blues'),
                 ('Six',            16,      8,       6, 'Classical',    'General');

select * from Item;

SELECT Category, COUNT(*) AS Total
FROM Item
WHERE Department='Popular'
GROUP BY Category
HAVING Total < 3;

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Use COUNT, GROUP and HAVING : Count « Select Clause « SQL / MySQL

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