Create a summary result with one query

问题

I have a table with the following format.

mysql> describe unit_characteristics;
+----------------------+------------------+------+-----+---------+----------------+
| Field                | Type             | Null | Key | Default | Extra          |
+----------------------+------------------+------+-----+---------+----------------+
| id                   | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| uut_id               | int(10) unsigned | NO   | PRI | NULL    |                |
| uut_sn               | varchar(45)      | NO   |     | NULL    |                |
| characteristic_name  | varchar(80)      | NO   | PRI | NULL    |                |
| characteristic_value | text             | NO   |     | NULL    |                |
| creation_time        | datetime         | NO   |     | NULL    |                |
| last_modified_time   | datetime         | NO   |     | NULL    |                |
+----------------------+------------------+------+-----+---------+----------------+

each uut_sn has multiple characteristic_name/value pairs. I want to use MySQL to generate a table

+----------------------+-------------+-------------+-------------+--------------+
| uut_sn | char_name_1 | char_name_2 | char_name_3 | char_name_4 | ...          |
+----------------------+-------------+-------------+-------------+--------------+
| 00000  | char_val_1  | char_val_2  | char_val_3  | char_val_4  | ...          | 
| 00001  | char_val_1  | char_val_2  | char_val_3  | char_val_4  | ...          |
| 00002  | char_val_1  | char_val_2  | char_val_3  | char_val_4  | ...          |
| .....  | char_val_1  | char_val_2  | char_val_3  | char_val_4  | ...          |
+----------------------+------------------+------+-----+---------+--------------+

Is this possible with just one query?

Thanks, -peter

回答1:

This is a standard pivot query:

  SELECT uc.uut_sn,
         MAX(CASE 
               WHEN uc.characteristic_name = 'char_name_1' THEN uc.characteristic_value 
               ELSE NULL 
             END) AS char_name_1,
         MAX(CASE 
               WHEN uc.characteristic_name = 'char_name_2' THEN uc.characteristic_value 
               ELSE NULL 
             END) AS char_name_2,
         MAX(CASE 
               WHEN uc.characteristic_name = 'char_name_3' THEN uc.characteristic_value 
               ELSE NULL 
             END) AS char_name_3,
    FROM unit_characteristics uc
GROUP BY uc.uut_sn

To make it dynamic, you need to use MySQL's dynamic SQL syntax called Prepared Statements. It requires two queries - the first gets a list of the characteristic_name values, so you can concatenate the appropriate string into the CASE expressions like you see in my example as the ultimate query.

回答2:

You're using the EAV antipattern. There's no way to automatically generate the pivot table you describe, without hardcoding the characteristics you want to include. As @OMG Ponies mentions, you need to use dynamic SQL to general the query in a custom fashion for the set of characteristics you want to include in the result.

Instead, I recommend you fetch the characteristics one per row, as they are stored in the database, and if you want an application object to represent a single UUT with all its characteristics, you write code to loop over the rows as you fetch them in your application, collecting them into objects.

For example in PHP:

$sql = "SELECT uut_sn, characteristic_name, characteristic_value 
        FROM unit_characteristics";
$stmt = $pdo->query($sql);

$objects = array();
while ($row = $stmt->fetch()) {
  if (!isset($objects[ $row["uut_sn"] ])) {
      $object[ $row["uut_sn"] ] = new Uut();
  }
  $objects[ $row["uut_sn"] ]->$row["characteristic_name"] 
                            = $row["characterstic_value"];
}

This has a few advantages over the solution of hardcoding characteristic names in your query:

This solution takes only one SQL query instead of two.
No complex code is needed to build your dynamic SQL query.
If you forget one of the characteristics, this solution automatically finds it anyway.
GROUP BY in MySQL is often slow, and this avoids the GROUP BY.

来源：https://stackoverflow.com/questions/3515286/create-a-summary-result-with-one-query

标签

sql

mysql

pivot

entity-attribute-value