Split column values into multiple columns with CREATE VIEW

问题

With a MySQL query, how can I take a table like in Example A:

Example A
+------+---------------+----------+
| id   | value         | class    |
+------+---------------+----------+
| 1    | 33.00         | total    |
| 1    | 12.00         | shipping |
| 2    | 45.00         | total    |
| 2    | 15.00         | shipping |
+------+---------------+----------+

And create a view like Example B?

Example B
+------+---------------+---------------+
| id   | value_total   | value_shipping|
+------+---------------+---------------+
| 1    | 33.00         | 12.00         |
| 2    | 45.00         | 15.00         |
+------+---------------+---------------+

回答1:

You can simply use SUM() function for that:

SELECT id
,SUM(CASE WHEN class = 'total' THEN value ELSE 0 END) AS value_total
,SUM(CASE WHEN class = 'shipping' THEN value ELSE 0 END) AS value_shipping
FROM Table1 
GROUP BY id;

See this SQLFiddle

If you have unknown number of class then try this Dynamic query

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(case when class = ''',
      class,
      ''' then value else 0 end) AS `value_',
      class, '`'
    )
  ) INTO @sql
FROM Table1;


SET @sql = CONCAT('SELECT id, ', @sql, '
                  FROM Table1 
                  GROUP BY id');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Output:

╔════╦═════════════╦════════════════╗
║ ID ║ VALUE_TOTAL ║ VALUE_SHIPPING ║
╠════╬═════════════╬════════════════╣
║  1 ║          33 ║             12 ║
║  2 ║          45 ║             15 ║
╚════╩═════════════╩════════════════╝

See this SQLFiddle

回答2:

try this

  select id , 
  max(case when class = 'total' then value end) as value_total
 , max(case when class = 'shipping' then value end) as value_shipping
  from Table1
  group by id

DEMO HERE

来源：https://stackoverflow.com/questions/16397666/split-column-values-into-multiple-columns-with-create-view

标签

mysql

sql

create-view