SQL GROUP BY

GROUP BY 語句可結(jié)合一些聚合函數(shù)來使用

GROUP BY 語句

GROUP BY 語句用于結(jié)合聚合函數(shù)，根據(jù)一個或多個列對結(jié)果集進行分組。

SQL GROUP BY 語法

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name;

演示數(shù)據(jù)庫

在本教程中，我們將使用 JSON 樣本數(shù)據(jù)庫。

下面是選自 "Websites" 表的數(shù)據(jù)：

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘寶          | https://www.taobao.com/   | 13    | CN      |
| 3  | 小白教程      | http://9t9t.com.cn    | 888  | CN      |
| 4  | 微博          | http://weibo.com/         | 20    | CN      |
| 5  | Facebook     | https://www.facebook.com/ | 3     | USA     |
| 7  | stackoverflow | http://stackoverflow.com/ |   0 | IND     |
+----+---------------+---------------------------+-------+---------+

下面是 "access_log" 網(wǎng)站訪問記錄表的數(shù)據(jù)：

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 | 2016-05-16 |
|   9 |       3 |   201 | 2016-05-17 |
+-----+---------+-------+------------+
9 rows in set (0.00 sec)

GROUP BY 簡單應(yīng)用

統(tǒng)計 access_log 各個 site_id 的訪問量：

實例

SELECT site_id, SUM(access_log.count) AS nums
FROM access_log GROUP BY site_id;

執(zhí)行以上 SQL 輸出結(jié)果如下：

SQL GROUP BY 多表連接

下面的 SQL 語句統(tǒng)計有記錄的網(wǎng)站的記錄數(shù)量：

實例

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;

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