首页sql升序和降序哪个更能优化查询语句的速度

sql升序和降序哪个更能优化查询语句的速度

时间: 2024-01-26 15:02:26 浏览: 107

采用降序排序-SQL语言基础

采用降序排序 * SQL> SELECT ename, job, deptno, hiredate 2 FROM emp 3 ORDER BY hiredate DESC; ENAME JOB DEPTNO HIREDATE ---------- --------- --------- --------- ADAMS CLERK 20 12-JAN-83 SCOTT ANALYST 20 09-DEC-82 MILLER CLERK 10 23-JAN-82 JAMES CLERK 30 03-DEC-81 FORD ANALYST 20 03-DEC-81 KING PRESIDENT 10 17-NOV-81 MARTIN SALESMAN 30 28-SEP-81 ... 14 rows selected. * SELECT ename, job, deptno, hiredate FROM emp ORDER BY hiredate DESC; Default Ordering of Data The default sort order is ascending: Numeric values are displayed with the lowest values first—for example, 1–999. Date values are displayed with the earliest value first—for example, 01-JAN-92 before 01-JAN-95. Character values are displayed in alphabetical order—for example, A first and Z last. Null values are displayed last for ascending sequences and first for descending sequences. Reversing the Default Order To reverse the order in which rows are displayed, specify the keyword DESC after the column name in the ORDER BY clause. The slide example sorts the result by the most recently hired employee. Instructor Note Let the students know that you can also sort by a column number in the SELECT list. The example below will sort the output in the descending order by salary. SQL> SELECT ename, sal 2 FROM emp 3 ORDER BY 2 DESC;

SQL查询的速度和排序方式并没有直接关系，升序和降序都是可以优化查询语句的速度的。但是，对于一些特定的场景，选择合适的排序方式可以让查询语句更加高效。例如，在使用索引进行排序时，如果按照索引的升序进行排序，可以让数据库系统更好地利用索引进行查询，从而提高查询效率。此外，如果需要对查询结果进行分页显示，使用升序排序可能比降序排序更加高效，因为数据库系统可以利用索引逆序扫描，从而更快地定位到需要的数据。综上所述，对于具体的场景需要具体分析，选择合适的排序方式来优化查询语句的速度。

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