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3.3.4.8 Counting Rows

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Databases are often used to answer the question, “How often does a certain type of data occur in a table?” For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of census operations on your animals.

Counting the total number of animals you have is the same question as “How many rows are in the pet table?” because there is one record per pet. COUNT(*) counts the number of rows, so the query to count your animals looks like this:

  1. mysql> SELECT COUNT(*) FROM pet;
  2. +----------+
  3. | COUNT(*) |
  4. +----------+
  5. |        9 |
  6. +----------+

Earlier, you retrieved the names of the people who owned pets. You can use COUNT() if you want to find out how many pets each owner has:

  1. mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
  2. +--------+----------+
  3. | owner  | COUNT(*) |
  4. +--------+----------+
  5. | Benny  |        2 |
  6. | Diane  |        2 |
  7. | Gwen   |        3 |
  8. | Harold |        2 |
  9. +--------+----------+

The preceding query uses GROUP BY to group all records for each owner. The use of COUNT() in conjunction with GROUP BY is useful for characterizing your data under various groupings. The following examples show different ways to perform animal census operations.

Number of animals per species:

  1. mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
  2. +---------+----------+
  3. | species | COUNT(*) |
  4. +---------+----------+
  5. | bird    |        2 |
  6. | cat     |        2 |
  7. | dog     |        3 |
  8. | hamster |        1 |
  9. | snake   |        1 |
  10. +---------+----------+

Number of animals per sex:

  1. mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
  2. +------+----------+
  3. | sex  | COUNT(*) |
  4. +------+----------+
  5. | NULL |        1 |
  6. | f    |        4 |
  7. | m    |        4 |
  8. +------+----------+

(In this output, NULL indicates that the sex is unknown.)

Number of animals per combination of species and sex:

  1. mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
  2. +---------+------+----------+
  3. | species | sex  | COUNT(*) |
  4. +---------+------+----------+
  5. | bird    | NULL |        1 |
  6. | bird    | f    |        1 |
  7. | cat     | f    |        1 |
  8. | cat     | m    |        1 |
  9. | dog     | f    |        1 |
  10. | dog     | m    |        2 |
  11. | hamster | f    |        1 |
  12. | snake   | m    |        1 |
  13. +---------+------+----------+

You need not retrieve an entire table when you use COUNT(). For example, the previous query, when performed just on dogs and cats, looks like this:

  1. mysql> SELECT species, sex, COUNT(*) FROM pet
  2.        WHERE species = 'dog' OR species = 'cat'
  3.        GROUP BY species, sex;
  4. +---------+------+----------+
  5. | species | sex  | COUNT(*) |
  6. +---------+------+----------+
  7. | cat     | f    |        1 |
  8. | cat     | m    |        1 |
  9. | dog     | f    |        1 |
  10. | dog     | m    |        2 |
  11. +---------+------+----------+

Or, if you wanted the number of animals per sex only for animals whose sex is known:

  1. mysql> SELECT species, sex, COUNT(*) FROM pet
  2.        WHERE sex IS NOT NULL
  3.        GROUP BY species, sex;
  4. +---------+------+----------+
  5. | species | sex  | COUNT(*) |
  6. +---------+------+----------+
  7. | bird    | f    |        1 |
  8. | cat     | f    |        1 |
  9. | cat     | m    |        1 |
  10. | dog     | f    |        1 |
  11. | dog     | m    |        2 |
  12. | hamster | f    |        1 |
  13. | snake   | m    |        1 |
  14. +---------+------+----------+

If you name columns to select in addition to the COUNT() value, a GROUP BY clause should be present that names those same columns. Otherwise, the following occurs:

  • If the ONLY_FULL_GROUP_BY SQL mode is enabled, an error occurs:

    1. mysql> SET sql_mode = 'ONLY_FULL_GROUP_BY';
    2. Query OK, 0 rows affected (0.00 sec)
    3.  
    4. mysql> SELECT owner, COUNT(*) FROM pet;
    5. ERROR 1140 (42000): In aggregated query without GROUP BY, expression
    6. #1 of SELECT list contains nonaggregated column 'menagerie.pet.owner';
    7. this is incompatible with sql_mode=only_full_group_by

  • If ONLY_FULL_GROUP_BY is not enabled, the query is processed by treating all rows as a single group, but the value selected for each named column is nondeterministic. The server is free to select the value from any row:

    1. mysql> SET sql_mode = '';
    2. Query OK, 0 rows affected (0.00 sec)
    3.  
    4. mysql> SELECT owner, COUNT(*) FROM pet;
    5. +--------+----------+
    6. | owner  | COUNT(*) |
    7. +--------+----------+
    8. | Harold |        8 |
    9. +--------+----------+
    10. 1 row in set (0.00 sec)

See also Section 12.20.3, “MySQL Handling of GROUP BY”. See Section 12.20.1, “Aggregate (GROUP BY) Function Descriptions” for information about COUNT(expr) behavior and related optimizations.


Rechercher dans le manuel MySQL

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