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。
HAVING Clause allows you to specify conditions to filter the grouped results that will appear in the final result.
The following is In a query, the HAVING clause must be placed after the GROUP BY clause and before the ORDER BY clause. The following is the syntax of the SELECT statement that contains the HAVING clause: Hypothetical The following is an example that displays all records with a name count less than 2: This will produce the following results: The following is an example that displays all records with a name count greater than 2: This will produce the following results: WHERE Clause sets the condition on the selected column, while the HAVING The clause is determined by the GROUP BY Sets the condition on the grouping created by the clause. 1.25.1. Grammar ¶
HAVING Clause in SELECT Location in the query:SELECT FROM WHERE GROUP BY HAVING ORDER BY
SELECT column1, column2 FROM table1, table2 WHERE [ conditions ] GROUP BY column1, column2 HAVING [ conditions ] ORDER BY column1, column2
1.25.2. 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
sqlite > SELECT * FROM COMPANY GROUP BY name HAVING count(name) < 2;
ID NAME AGE ADDRESS SALARY ---------- ---------- ---------- ---------- ---------- 2 Allen 25 Texas 15000 5 David 27 Texas 85000 6 Kim 22 South-Hall 45000 4 Mark 25 Rich-Mond 65000 3 Teddy 23 Norway 20000
sqlite > SELECT * FROM COMPANY GROUP BY name HAVING count(name) > 2;
ID NAME AGE ADDRESS SALARY ---------- ---------- ---------- ---------- ---------- 10 James 45 Texas 5000