SQL GROUP BY
SQL GROUP BY Statement
GROUP BY statement can be used in conjunction with some aggregate functions
GROUP BY statement
The GROUP BY statement is used in combination with aggregate functions to group the result set based on one or more columns.
SQL GROUP BY syntax
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;
FROM table_name
WHERE column_name operator value
GROUP BY column_name;
Demo Database
In this tutorial, we will use the php sample database.
The following is the data selected from the "Websites" table:
+----+--------------+--- ------------------------+------+---------+
| id | name | url --------+-------+---------+
| 1 | Google | https://www.google.cm/ | 1 | USA |
| 2 | Taobao | https://www.taobao.com/ | 13 | CN |
| 3 | php Chinese website | http://www.php.cn/ | 4689 | CN |
| 4 | Weibo | http://weibo.com/ | 20 | CN |
| 5 | Facebook | https://www.facebook.com/ | 3 | USA |
| 7 | stackoverflow | http://stackoverflow.com/ | 0 | IND |
+----+---------------+----------- ----------------+------+---------+
| id | name | url --------+-------+---------+
| 1 | Google | https://www.google.cm/ | 1 | USA |
| 2 | Taobao | https://www.taobao.com/ | 13 | CN |
| 3 | php Chinese website | http://www.php.cn/ | 4689 | CN |
| 4 | Weibo | http://weibo.com/ | 20 | CN |
| 5 | Facebook | https://www.facebook.com/ | 3 | USA |
| 7 | stackoverflow | http://stackoverflow.com/ | 0 | IND |
+----+---------------+----------- ----------------+------+---------+
The following is the data of the "access_log" website access record table:
mysql> SELECT * FROM access_log;
+-----+--------- +-------+------------+
| aid | site_id | count | date |
+-----+------ ---+-------+------------+
| 1 | 1 | 45 | 2016-05-10 |
| 2 | 3 | 100 | 2016-05-13 |
| 3 | 1 | 230 | 2016-05-14 |
| 4 | 2 | 10 | 2016-05-14 |
| 5 | 5 | 205 | 2016 -05-14 |
| 6 | 4 | 13 | 2016-05-15 |
| 7 | 3 | 220 | 2016-05-15 |
| 8 | 5 | 545 | 201 6-05 -16 |
| 9 | 3 | 201 | 2016-05-17 |
+-----+----------+------+--- ---------+
9 rows in set (0.00 sec)
+-----+--------- +-------+------------+
| aid | site_id | count | date |
+-----+------ ---+-------+------------+
| 1 | 1 | 45 | 2016-05-10 |
| 2 | 3 | 100 | 2016-05-13 |
| 3 | 1 | 230 | 2016-05-14 |
| 4 | 2 | 10 | 2016-05-14 |
| 5 | 5 | 205 | 2016 -05-14 |
| 6 | 4 | 13 | 2016-05-15 |
| 7 | 3 | 220 | 2016-05-15 |
| 8 | 5 | 545 | 201 6-05 -16 |
| 9 | 3 | 201 | 2016-05-17 |
+-----+----------+------+--- ---------+
9 rows in set (0.00 sec)
GROUP BY Simple application
Statistics access_log visits to each site_id ：
Example
SELECT site_id, SUM(access_log.count) AS
nums
FROM access_log GROUP BY site_id;
FROM access_log GROUP BY site_id;
Execute the above SQL and the output result is as follows:
##SQL GROUP BY Multi-table connectionNow we want to find the number of orders delivered by each deliveryman. The following SQL statement counts the number of records visited by all websites:
Example
SELECT Websites.name,COUNT(access_log.aid) AS nums FROM
access_log
LEFT JOIN Websites
ON access_log.site_id=Websites.id
GROUP BY Websites.name;
Execute the above SQL and the output result is as follows:ON access_log.site_id=Websites.id
GROUP BY Websites.name;
