数据库优化第三讲（数据的修改和删除及数据的条件查询））

数据库优化

标题

修改
– update 表名 set 列1=值1，列2=值2… where 条件；
– 修改 名字为juran 改成 煎饼侠
update students set name=‘煎饼侠’; – 不加where 全部修改
update students set name=‘python’ where name=‘煎饼侠’;

update students set name=‘无名’ where id=6; – 修改id 为6的
update students set name=‘帅哥’,gender = ‘保密’ where id=7;

删除
物理删除
–delete from 表名 where 条件
delete from students; --删除的是 数据
delete from students where id =1;

逻辑删除
– is_delete 表示 是否删除 1 删除 0 未删除
– 查找 select * from students where is_delete=0;
– 修改
update students set is_delete = 1 where id=6;

查询基本使用
– select 去重选项 字段列表[as 字段名] from 数据表
– where [group by 子句] [having子句] [order by 子句] [limit子句];

–查询所有列
select * from students;

– 去除重复字段的查询 distinct 整个查询的行是否重复
select distinct name from students;

–查询指定列
– select 列1，列2，… from 表名；
select name,age,gender from students;

– 可以使用as为列或表指定别名
select name as n,age,gender from students_abc_demoas_asdasd where n = ‘juran’;
select name as n,gender from students where name=‘帅哥’;
select name as 姓名,gender from students where name=‘帅哥’;