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。
In PostgreSQL DISTINCT Keywords vs. SELECT Statement to remove duplicate records and get only unique records.
When we are usually manipulating data, there may be a situation in which there are multiple duplicate records in a table, and when extracting such records Used to remove duplicate records Create Let’s insert two pieces of data: Now the figures are as follows: Next, let’s find all the NAME in the COMPANY table: The results are as follows: Now we’re here. The results are as follows: As you can see from the results, the duplicate data has been deleted. DISTINCT The keyword is particularly meaningful because it only gets a single record, not a duplicate record. 5.27.1. Grammar ¶
DISTINCT The basic syntax for keywords is as follows:SELECT DISTINCT column1, column2,.....columnN FROM table_name WHERE [condition]
5.27.2. Example ¶
COMPANY 表（ 下载 COMPANY SQL 文件 ), the data are as follows:runoobdb# select * from COMPANY; id | name | age | address | salary ----+-------+-----+-----------+-------- 1 | Paul | 32 | California| 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 6 | Kim | 22 | South-Hall| 45000 7 | James | 24 | Houston | 10000 (7 rows)
INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) VALUES (8, 'Paul', 32, 'California', 20000.00 ); INSERT INTO COMPANY (ID,NAME,AGE,ADDRESS,SALARY) VALUES (9, 'Allen', 25, 'Texas', 15000.00 );
id | name | age | address | salary ----+-------+-----+------------+-------- 1 | Paul | 32 | California | 20000 2 | Allen | 25 | Texas | 15000 3 | Teddy | 23 | Norway | 20000 4 | Mark | 25 | Rich-Mond | 65000 5 | David | 27 | Texas | 85000 6 | Kim | 22 | South-Hall | 45000 7 | James | 24 | Houston | 10000 8 | Paul | 32 | California | 20000 9 | Allen | 25 | Texas | 15000 (9 rows)
runoobdb=# SELECT name FROM COMPANY;
name ------- Paul Allen Teddy Mark David Kim James Paul Allen (9 rows)
SELECT Statement using the DISTINCT Clause:runoobdb=# SELECT DISTINCT name FROM COMPANY;
name ------- Teddy Paul Mark David Allen Kim James (7 rows)