最近几年来，地理信息系统无论是在理论上还是应用上都处在一个飞速发展的阶段。 GIS被应用于多个领域的建模和决策支持，如城市管理、区划、环境整治等等，地理信息成为信息时代重要的组成部分之一； “数字地球”概念的提出，更进一步推动了作为其技术支撑的GIS的发展。 与此同时，一些学者致力于相关的理论研究，如空间感知、空间数据误差、空间关系的形式化等等。 这恰好说明了地理信息系统作为应用技术和学科的两个方面，并且这两个方面构成了相互促进的发展过程。
The GROUP BY statement groups the result set according to one or more columns.
We can use COUNT, SUM, AVG, and other functions on the grouped columns. The example of this chapter uses the following table structure and data. We can import the following data into the database before using it. After the import is successful, execute the following SQL statement: Next, we use the GROUP BY statement to group the data tables by name and count how many records each person has: WITH ROLLUP can realize the same statistics on the basis of grouped statistics (SUM,AVG,COUNT. ). For example, we group the above data tables by name, and then count the number of logins for each person: Where the record NULL represents the number of logins for everyone. We can use coalesce to set a name that can replace NUll, coalesce syntax: Parameter description: select b if a==null, c if b==null, an if a b c is null, and null if both a b c are null. In the following example, if the name is empty, we use the total instead:GROUP BY syntax ¶
SELECT column_name, function(column_name) FROM table_name WHERE column_name operator value GROUP BY column_name;
Example demonstration ¶
SET NAMES utf8; SET FOREIGN_KEY_CHECKS = 0; -- ---------------------------- -- Table structure for `employee_tbl` -- ---------------------------- DROP TABLE IF EXISTS `employee_tbl`; CREATE TABLE `employee_tbl` ( `id` int(11) NOT NULL, `name` char(10) NOT NULL DEFAULT '', `date` datetime NOT NULL, `signin` tinyint(4) NOT NULL DEFAULT '0' COMMENT '登录次数', PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of `employee_tbl` -- ---------------------------- BEGIN; INSERT INTO `employee_tbl` VALUES ('1', '小明', '2016-04-22 15:25:33', '1'), ('2', '小王', '2016-04-20 15:25:47', '3'), ('3', '小丽', '2016-04-19 15:26:02', '2'), ('4', '小王', '2016-04-07 15:26:14', '4'), ('5', '小明', '2016-04-11 15:26:40', '4'), ('6', '小明', '2016-04-04 15:26:54', '2'); COMMIT; SET FOREIGN_KEY_CHECKS = 1; mysql> set names utf8; mysql> SELECT * FROM employee_tbl; +----+--------+---------------------+--------+ | id | name | date | signin | +----+--------+---------------------+--------+ | 1 | 小明 | 2016-04-22 15:25:33 | 1 | | 2 | 小王 | 2016-04-20 15:25:47 | 3 | | 3 | 小丽 | 2016-04-19 15:26:02 | 2 | | 4 | 小王 | 2016-04-07 15:26:14 | 4 | | 5 | 小明 | 2016-04-11 15:26:40 | 4 | | 6 | 小明 | 2016-04-04 15:26:54 | 2 | +----+--------+---------------------+--------+ 6 rows in set (0.00 sec)
mysql> SELECT name, COUNT(*) FROM employee_tbl GROUP BY name; +--------+----------+ | name | COUNT(*) | +--------+----------+ | 小丽 | 1 | | 小明 | 3 | | 小王 | 2 | +--------+----------+ 3 rows in set (0.01 sec)
Use WITH ROLLUP ¶
mysql> SELECT name, SUM(signin) as signin_count FROM employee_tbl GROUP BY name WITH ROLLUP; +--------+--------------+ | name | signin_count | +--------+--------------+ | 小丽 | 2 | | 小明 | 7 | | 小王 | 7 | | NULL | 16 | +--------+--------------+ 4 rows in set (0.00 sec)
select coalesce(a,b,c);
mysql> SELECT coalesce(name, '总数'), SUM(signin) as signin_count FROM employee_tbl GROUP BY name WITH ROLLUP; +--------------------------+--------------+ | coalesce(name, '总数') | signin_count | +--------------------------+--------------+ | 小丽 | 2 | | 小明 | 7 | | 小王 | 7 | | 总数 | 16 | +--------------------------+--------------+ 4 rows in set (0.01 sec)