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。
SQLite’s DISTINCT Keywords vs. SELECT Statement to eliminate all duplicate records and get only one record.
It is possible to have multiple duplicate records in a table. When extracting such a record, the Used to eliminate duplicate records Hypothetical First, let’s take a look at the following This will produce the following results: Now, let’s look at the above This will produce the following results without any duplicate entries: DISTINCT keyword is particularly meaningful, which only gets the only record, not the duplicate record. 1.26.1. Grammar ¶
DISTINCT The basic syntax for keywords is as follows:SELECT DISTINCT column1, column2,.....columnN FROM table_name WHERE [condition]
1.26.2. Example ¶
COMPANY The table has the following records:ID NAME AGE ADDRESS SALARY ---------- ---------- ---------- ---------- ---------- 1 Paul 32 California 20000.0 2 Allen 25 Texas 15000.0 3 Teddy 23 Norway 20000.0 4 Mark 25 Rich-Mond 65000.0 5 David 27 Texas 85000.0 6 Kim 22 South-Hall 45000.0 7 James 24 Houston 10000.0 8 Paul 24 Houston 20000.0 9 James 44 Norway 5000.0 10 James 45 Texas 5000.0
SELECT Query, which returns duplicate payroll records:sqlite> SELECT name FROM COMPANY;
NAME ---------- Paul Allen Teddy Mark David Kim James Paul James James
SELECT Used in query DISTINCT Keywords:sqlite> SELECT DISTINCT name FROM COMPANY;
NAME ---------- Paul Allen Teddy Mark David Kim James