MySQL查询有效地选择前n行？
使用索引可以有效地选择前n行。让我们首先创建一个表-
create table DemoTable (StudentName varchar(100), StudentScore int );
示例
使用插入命令在表中插入一些记录-
insert into DemoTable values('John',34);
insert into DemoTable values('Carol',55);
insert into DemoTable values('Bob',58);
insert into DemoTable values('Sam',38);
insert into DemoTable values('Mike',48);
insert into DemoTable values('Adam',41);
insert into DemoTable values('Chris',47);
insert into DemoTable values('Robert',40);
insert into DemoTable values('David',89);使用select语句显示表中的所有记录-
select *from DemoTable;
输出结果
+-------------+--------------+ | StudentName | StudentScore | +-------------+--------------+ | John | 34 | | Carol | 55 | | Bob | 58 | | Sam | 38 | | Mike | 48 | | Adam | 41 | | Chris | 47 | | Robert | 40 | | David | 89 | +-------------+--------------+ 9 rows in set (0.00 sec)
示例
以下是有效选择前n行的查询。我们使用了ORDERBY并跳过了5行。跳过后，有3条记录可见，因为我们使用了LIMIT3-
alter table DemoTable ADD INDEX name_score(StudentName,StudentScore); Records: 0 Duplicates: 0 Warnings: 0 select StudentName,StudentScore from DemoTable order by StudentScore LIMIT 5,3;
输出结果
+-------------+--------------+ | StudentName | StudentScore | +-------------+--------------+ | Mike | 48 | | Carol | 55 | | Bob | 58 | +-------------+--------------+ 3 rows in set (0.00 sec)