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12.18.2 GROUP BY Modifiers

The GROUP BY clause permits a WITH ROLLUP modifier that causes summary output to include extra rows that represent higher-level (that is, super-aggregate) summary operations. ROLLUP thus enables you to answer questions at multiple levels of analysis with a single query. For example, ROLLUP can be used to provide support for OLAP (Online Analytical Processing) operations.

Suppose that a sales table has year, country, product, and profit columns for recording sales profitability:

CREATE TABLE sales
(
    year    INT,
    country VARCHAR(20),
    product VARCHAR(32),
    profit  INT
);

To summarize table contents per year, use a simple GROUP BY like this:

mysql> SELECT year, SUM(profit) AS profit
       FROM sales
       GROUP BY year ASC;
+------+--------+
| year | profit |
+------+--------+
| 2000 |   4525 |
| 2001 |   3010 |
+------+--------+

The output shows the total profit for each year. To also determine the total profit summed over all years, you must add up the individual values yourself or run an additional query. Or you can use ROLLUP, which provides both levels of analysis with a single query. Adding a WITH ROLLUP modifier to the GROUP BY clause causes the query to produce another row that shows the grand total over all year values:

mysql> SELECT year, SUM(profit) AS profit
       FROM sales
       GROUP BY year ASC WITH ROLLUP;
+------+--------+
| year | profit |
+------+--------+
| 2000 |   4525 |
| 2001 |   3010 |
| NULL |   7535 |
+------+--------+

The NULL value in the year column identifies the grand total super-aggregate line.

ROLLUP has a more complex effect when there are multiple GROUP BY columns. In this case, each time there is a change in value in any but the last grouping column, the query produces an extra super-aggregate summary row.

For example, without ROLLUP, a summary of the sales table based on year, country, and product might look like this, where the output indicates summary values only at the year/country/product level of analysis:

mysql> SELECT year, country, product, SUM(profit) AS profit
       FROM sales
       GROUP BY year ASC, country ASC, product ASC;
+------+---------+------------+--------+
| year | country | product    | profit |
+------+---------+------------+--------+
| 2000 | Finland | Computer   |   1500 |
| 2000 | Finland | Phone      |    100 |
| 2000 | India   | Calculator |    150 |
| 2000 | India   | Computer   |   1200 |
| 2000 | USA     | Calculator |     75 |
| 2000 | USA     | Computer   |   1500 |
| 2001 | Finland | Phone      |     10 |
| 2001 | USA     | Calculator |     50 |
| 2001 | USA     | Computer   |   2700 |
| 2001 | USA     | TV         |    250 |
+------+---------+------------+--------+

With ROLLUP added, the query produces several extra rows:

mysql> SELECT year, country, product, SUM(profit) AS profit
       FROM sales
       GROUP BY year ASC, country ASC, product ASC WITH ROLLUP;
+------+---------+------------+--------+
| year | country | product    | profit |
+------+---------+------------+--------+
| 2000 | Finland | Computer   |   1500 |
| 2000 | Finland | Phone      |    100 |
| 2000 | Finland | NULL       |   1600 |
| 2000 | India   | Calculator |    150 |
| 2000 | India   | Computer   |   1200 |
| 2000 | India   | NULL       |   1350 |
| 2000 | USA     | Calculator |     75 |
| 2000 | USA     | Computer   |   1500 |
| 2000 | USA     | NULL       |   1575 |
| 2000 | NULL    | NULL       |   4525 |
| 2001 | Finland | Phone      |     10 |
| 2001 | Finland | NULL       |     10 |
| 2001 | USA     | Calculator |     50 |
| 2001 | USA     | Computer   |   2700 |
| 2001 | USA     | TV         |    250 |
| 2001 | USA     | NULL       |   3000 |
| 2001 | NULL    | NULL       |   3010 |
| NULL | NULL    | NULL       |   7535 |
+------+---------+------------+--------+

Now the output includes summary information at four levels of analysis, not just one:

  • Following each set of product rows for a given year and country, an extra summary row appears showing the total for all products. These rows have the product column set to NULL.

  • Following each set of rows for a given year, an extra summary row appears showing the total for all countries and products. These rows have the country and products columns set to NULL.

  • Finally, following all other rows, an extra summary row appears showing the grand total for all years, countries, and products. This row has the year, country, and products columns set to NULL.

Other Considerations When using ROLLUP

The following discussion lists some behaviors specific to the MySQL implementation of ROLLUP.

When you use ROLLUP, you cannot also use an ORDER BY clause to sort the results. In other words, ROLLUP and ORDER BY are mutually exclusive in MySQL. However, you still have some control over sort order. GROUP BY in MySQL implicitly sorts results by default (in the absence of ASC or DESC designators). However, implicit GROUP BY sorting in MySQL is deprecated. To achieve a specific sort order of grouped results:

  • Use explicit ASC and DESC keywords with columns named in the GROUP BY list to specify sort order for individual columns. In this case, the super-aggregate summary rows added by ROLLUP still appear after the rows from which they are calculated, regardless of the sort order.

  • To work around the restriction that prevents using ROLLUP with ORDER BY, generate the grouped result set as a derived table and apply ORDER BY to it. For example:

    SELECT * FROM
    (SELECT year, SUM(profit) FROM sales GROUP BY year WITH ROLLUP) AS dt
    ORDER BY year;
    

    In this case, the super-aggregate summary rows sort with the rows from which they are calculated, and their placement depends on sort order (at the beginning for ascending sort, at the end for descending sort).

LIMIT can be used to restrict the number of rows returned to the client. LIMIT is applied after ROLLUP, so the limit applies against the extra rows added by ROLLUP. For example:

mysql> SELECT year, country, product, SUM(profit) AS profit
       FROM sales
       GROUP BY year ASC, country ASC, product ASC WITH ROLLUP
       LIMIT 5;
+------+---------+------------+--------+
| year | country | product    | profit |
+------+---------+------------+--------+
| 2000 | Finland | Computer   |   1500 |
| 2000 | Finland | Phone      |    100 |
| 2000 | Finland | NULL       |   1600 |
| 2000 | India   | Calculator |    150 |
| 2000 | India   | Computer   |   1200 |
+------+---------+------------+--------+

Using LIMIT with ROLLUP may produce results that are more difficult to interpret, because there is less context for understanding the super-aggregate rows.

The NULL indicators in each super-aggregate row are produced when the row is sent to the client. The server looks at the columns named in the GROUP BY clause following the leftmost one that has changed value. For any column in the result set with a name that matches any of those names, its value is set to NULL. (If you specify grouping columns by column position, the server identifies which columns to set to NULL by position.)

Because the NULL values in the super-aggregate rows are placed into the result set at such a late stage in query processing, you can test them as NULL values only in the select list or HAVING clause. You cannot test them as NULL values in join conditions or the WHERE clause to determine which rows to select. For example, you cannot add WHERE product IS NULL to the query to eliminate from the output all but the super-aggregate rows.

The NULL values do appear as NULL on the client side and can be tested as such using any MySQL client programming interface. However, at this point, you cannot distinguish whether a NULL represents a regular grouped value or a super-aggregate value.

A MySQL extension permits a column that does not appear in the GROUP BY list to be named in the select list. (For information about nonaggregated columns and GROUP BY, see Section 12.18.3, “MySQL Handling of GROUP BY”.) In this case, the server is free to choose any value from this nonaggregated column in summary rows, and this includes the extra rows added by WITH ROLLUP. For example, in the following query, country is a nonaggregated column that does not appear in the GROUP BY list and values chosen for this column are indeterminate:

mysql> SELECT year, country, SUM(profit) AS profit
       FROM sales
       GROUP BY year ASC WITH ROLLUP;
+------+---------+--------+
| year | country | profit |
+------+---------+--------+
| 2000 | India   |   4525 |
| 2001 | USA     |   3010 |
| NULL | USA     |   7535 |
+------+---------+--------+

This behavior is permitted when the ONLY_FULL_GROUP_BY SQL mode is not enabled. If that mode is enabled, the server rejects the query as illegal because country is not listed in the GROUP BY clause.


User Comments
  Posted by Dan Nelson on May 2, 2007
Even though the NULL rows are inserted late in query processing, they seem to be available to the select_expr part of a SELECT statement. Using the example table:

SELECT IFNULL(year,"Total") as year, SUM(profit) FROM sales GROUP BY year WITH ROLLUP;

returns:

+-------+-------------+
| year | SUM(profit) |
+-------+-------------+
| 2000 | 4525 |
| 2001 | 3010 |
| Total | 7535 |
+-------+-------------+

  Posted by Parvesh Garg on July 23, 2007
NULL rows are inserted very late in the query process, but they can be used very well as described in the post above. Also they can be used with the HAVING clause.

mysql> SELECT IFNULL(url, 'ALL_URLS') AS url, IFNULL(year, 'ALL_YEARS') AS year, IFNULL(country, 'ALL_COUNTRIES') AS country, SUM(visit) FROM rollup_1 GROUP BY url, year, country WITH ROLLUP HAVING country is null;
+----------------------+-----------+---------------+------------+
| url | year | country | SUM(visit) |
+----------------------+-----------+---------------+------------+
| http://www.yahoo.com | 2005 | ALL_COUNTRIES | 22000 |
| http://www.yahoo.com | 2006 | ALL_COUNTRIES | 26700 |
| http://www.yahoo.com | 2007 | ALL_COUNTRIES | 34200 |
| http://www.yahoo.com | ALL_YEARS | ALL_COUNTRIES | 82900 |
| ALL_URLS | ALL_YEARS | ALL_COUNTRIES | 82900 |
+----------------------+-----------+---------------+------------+
5 rows in set, 4 warnings (0.00 sec)

There is one gotcha, if the column value itself has a NULL value, you may see undefined behavior.
  Posted by Ian Lewis on May 19, 2008
Instead of IFNULL(fieldname,"fieldData") as field it is possible to use COALESCE(fieldname,"fieldData") as field to do the same job.

As they are doing the same job it would preferable to use COALESCE as it is included in the ANSI SQL-92 standard.
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