查詢num行數:
select count(num) from info;
查詢num去重複後的行數:
1.select instinct num from info; (給num去重)
2.select count (instinct num) from info; (查詢去重後num的行數)
查找數據最大值和最小值:
select max(字段名) as 字段名MAX,min(字段名) as 字段名MIN;
計算平均值:select avg(score) from info;
計算總值:select sum(score) from info;
group by:按照...進行分組
select num,sname,gender group by gender; (按照gender進行分組)
加一列:alter table info add column 字段名 數據類型(字節);
減一列:alter table info drop column 字段名;
創建自增列:
create table info(id int auto_increament primary key,sname char(8));
兩表連接:
首先要求兩個表相引用的字段數據類型和約束相同
alter table stu add constraint foreign key(sid) references info(num);
兩表拼接:
select * from info inner join stu on info.num=stu.sid; 兩表相對應的列