原图
//进行乘法运算
select *,age*2 from user_student;
字符串做算术运算时,会被当做0来进行运算,在字符串中+号不代表拼接
//通过where条件查询user_student表下age大于22的数据
select id,name,age from user_student where age > 22;
//通过where条件查询 between 关键字设置范围查询年龄在20到30之间的数据
select id,name,age from user_student where age between 20 and 30;
// in 关键字 数据name是王五的数据
select id,name,age from user_student where name in('王五');
//模糊查询 下划线“_”可以匹配1个字符,如果要匹配0-n个字符,需要用“%”;
select * from user_student where name like '李_';
select * from user_student where name like '%四';
//按age排序,asc升序,desc降序,其中asc是可以省略的
select * from user_student order by age asc ;
select * from user_student order by age desc;
//分组查询 name出现几次 count(*) 计数
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select name, count(*)from user_student group by name;
//count(*) = 2
select name, count(*)from user_student group by name having count(*) = 2;
//分页查询 limit 1,2第二行开始 查询两行
select * from user_student limit 1,2;