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数据库查询优化指南

1. 基础查询

简单查询

select * from info;

select Code as '代号', name as '姓名' from info;

条件查询

select * from car where code = 'c002';

select * from car where brand = 'b001' or power = 130;

2. 模糊查询

select * from car where name like '%奥迪%';

注意：%表示任意多个字符，_表示单个字符。

3. 排序查询

select * from car order by brand, power desc;

默认排序为升序，不需要写asc。

4. 范围查询

select * from car where price between 40 and 60;

5. 离散查询

select * from car where code in('coo1','c003','c005','c007');

select * from car where code not in('coo1','c003','c005','c007');

6. 统计查询（聚合函数）

select count(code) from car; -- 查询记录数

select sum(price) from car; -- 查询总和

select max(code) from car; -- 查询最大值

select min(brand) from car; -- 查询最小值

select avg(price) from car; -- 查询平均值

select year(brithday) from dtudent; -- 提取生日的年份

7. 分页查询

select * from Car limit (n-1)*5,5; -- 取第n页，每页显示5条

8. 去重查询

select distinct Brand from Car;

9. 分组查询

select Brand, count(*) from Car group by Brand; -- 按品牌分组并统计记录数

select Brand from Car group by Brand having count(*) > 3; -- 分组后按条件筛选

10. 高级查询

连接查询（扩展字段）

select Info.Code, Info.Name, Info.Sex, Nation.Name, Info.Birthday from Info, Nation where Info.Nation = Nation.Code;

联合查询（扩展行）

select Code, Name from Infounionselect Code, Name from Nation;

子查询

无关子查询

select * from Info where Nation = (select Code from Nation where Name = '汉族');

相关子查询

select * from Car a where Oil < (select avg(Oil) from Car b where b.Brand = a.Brand);

注意事项

• 使用group by时，注意having语句进行筛选。
• 子查询通常用于条件限制或聚合函数中。
• 在like查询中，%和_的使用需要谨慎，避免过度匹配。

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