MySQL默认对ORDER BY使用什么排序规则？

Question

These queries both give the result I expect:

SELECT sex
FROM ponies
ORDER BY sex COLLATE latin1_swedish_ci ASC

SELECT sex
FROM ponies
ORDER BY CONVERT(sex USING utf8) COLLATE utf8_general_ci ASC

| f |
| f |
| m |
| m |
+---+

But this query gives a different result:

SELECT sex FROM ponies ORDER BY sex ASC

| m |
| m |
| f |
| f |
+---+

Here's the configuration:

SHOW VARIABLES LIKE 'collation\_%'

| collation_connection | utf8_general_ci   |
| collation_database   | latin1_swedish_ci |
| collation_server     | latin1_swedish_ci |
+----------------------+-------------------+

The table collation is latin1_swedish_ci .

MySQL server is 5.5.16.

Answer 1

Table Collations

Collation defaults are stored on a table-by-table basis. There is a server-set default, but that is applied to the table at the time it is created.

To find the collation for a specific table, run this query:

SHOW TABLE STATUS LIKE 'ponies'\G

You should see output like this:

*************************** 1. row ***************************
           Name: ponies
         Engine: MyISAM
        Version: 10
     Row_format: Fixed
           Rows: 8
 Avg_row_length: 20
    Data_length: 160
Max_data_length: 5629499534213119
   Index_length: 1024
      Data_free: 0
 Auto_increment: NULL
    Create_time: 2012-02-27 10:16:25
    Update_time: 2012-02-27 10:17:40
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)

And you can see the Collation setting in that result.

Column collations

You can also override collation settings on particular columns within a table. A create table statement like this would create a latin1_swedish_ci table, with a utf8_polish_ci column:

CREATE TABLE ponies (
    sex CHAR(1) COLLATE utf8_polish_ci
) CHARACTER SET latin1 COLLATE latin1_swedish_ci;

The best way to view the results of this is like this:

SHOW FULL COLUMNS FROM ponies;

Output:

+-------+---------+----------------+------+-----+---------+-------+---------------------------------+---------+
| Field | Type    | Collation      | Null | Key | Default | Extra | Privileges                      | Comment |
+-------+---------+----------------+------+-----+---------+-------+---------------------------------+---------+
| sex   | char(1) | utf8_polish_ci | YES  |     | NULL    |       | select,insert,update,references |         |
+-------+---------+----------------+------+-----+---------+-------+---------------------------------+---------+
1 row in set (0.00 sec)

Answer 2

The documentation says it uses a case insensitive character comparison by default. I don't see why you are not getting that result though.

The documentation also suggests using the binary qualifier for case sensitive comparison. I wonder if that would affect your result?:

SELECT sex FROM ponies ORDER BY BINARY sex ASC

Answer 3

This behaviour can be observed when sex is an ENUM in which case it is usually sorted by the numerical position in the ENUM definition. Only when a collation is explicitly given an it is sorted in alphabetical order.