GROUP BY returns a final sum for the first column and supplementary partial sums for the second column.

      
mysql>
mysql>
mysql>
mysql> CREATE TABLE titles (
    ->   titleID int(11),
    ->   title varchar(100),
    ->   subtitle varchar(100),
    ->   edition tinyint(4),
    ->   publID int(11),
    ->   catID int(11),
    ->   langID int(11),
    ->   year int(11),
    ->   isbn varchar(20),
    ->   comment varchar(255),
    ->   ts timestamp,
    ->   authors varchar(255),
    ->   PRIMARY KEY  (titleID)
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql>
mysql>
mysql> INSERT INTO titles VALUES (1,'Linux','Installation',5,1,57,2,2000,NULL,NULL,'2005-02-28 13:34:21','Michael'),
    ->                           (2,'Excel',NULL,NULL,2,3,NULL,2000,NULL,NULL,'2005-02-28 13:34:22','David'),
    ->                           (3,'XML',NULL,NULL,1,2,NULL,1997,NULL,NULL,'2005-02-28 13:34:22','Edwards'),
    ->                           (4,'PHP',NULL,NULL,3,6,NULL,2000,NULL,NULL,'2005-02-28 13:34:22','Tom'),
    ->                           (5,'MySQL','',0,3,34,NULL,2000,'','','2005-02-28 13:34:22','Paul'),
    ->                           (6,'Java',NULL,NULL,4,34,NULL,1999,NULL,NULL,'2005-02-28 13:34:22','Tim');
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql>
mysql> CREATE TABLE languages (
    ->   langID int(11) NOT NULL auto_increment PRIMARY KEY,
    ->   langName varchar(40),
    ->   ts timestamp
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql>
mysql>
mysql> INSERT INTO languages VALUES (1,'english','2004-12-02 18:37:02'),
    ->                                (2,'deutsch','2004-12-02 18:37:02'),
    ->                                (3,'svensk','2004-12-02 18:37:02'),
    ->                                (4,'norsk','2004-12-02 18:37:02');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql> CREATE TABLE categories (
    ->   catID int(11) NOT NULL auto_increment PRIMARY KEY ,
    ->   catName varchar(60),
    ->   parentCatID int(11),
    ->   ts timestamp
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql>
mysql>
mysql> INSERT INTO categories VALUES (1,'Computer books',11,'2004-12-02 18:37:20'),
    ->                                 (2,'Databases',1,'2004-12-02 18:37:20'),
    ->                                 (3,'Programming',1,'2004-12-02 18:37:20'),
    ->                                 (4,'Relational Databases',2,'2004-12-02 18:37:20'),
    ->                                 (5,'Object-oriented databases',2,'2004-12-02 18:37:20'),
    ->                                 (6,'PHP',3,'2004-12-02 18:37:20'),
    ->                                 (7,'Perl',3,'2004-12-02 18:37:20'),
    ->                                 (8,'SQL',2,'2004-12-02 18:37:20'),
    ->                                 (9,'Children\'s books',11,'2004-12-02 18:37:20'),
    ->                                 (10,'Literature and fiction',11,'2004-12-02 18:37:20'),
    ->                                 (11,'All books',NULL,'2004-12-02 18:37:20');
Query OK, 11 rows affected (0.00 sec)
Records: 11  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql>
mysql> SELECT langName, catName, COUNT(*)
    -> FROM titles, languages, categories
    -> WHERE titles.catID = categories.catID
    -> AND titles.langID = languages.langID
    -> GROUP BY langName, catName WITH ROLLUP;
Empty set (0.00 sec)

mysql>
mysql>
mysql> drop table titles;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table languages;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table categories;
Query OK, 0 rows affected (0.00 sec)

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35.	Dividing a Summary into Subgroups
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37.	Parentheses may be used to group alternations.
38.	To be more specific and find out how many messages each sender sent from each host, use two grouping columns.
39.	Categorize groups on a logarithmic scale.
40.	Missing and non-missing counts can be determined for subgroups as well.
41.	get the number of orders per customer
42.	A less fine-grained summary can be obtained by using only the month values
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GROUP BY returns a final sum for the first column and supplementary partial sums for the second column. : Group « Select Clause « SQL / MySQL

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