Question

我从我正在使用的MySql数据库中获取这些表：

create table employees (
    num_pass int(5) not null,
    name varchar(40),
    primary key (num_pass)
    )engine=innodb;

create table laboratories (
    code int(10) not null,
    name varchar(40),
    primary key (codi),
)engine=innodb;

create table areas (
    code int(5) not null,
    codeLab int(10) not null,
    level enum('H','M','L'),
    primary key (code, codeLab),
    foreign key (codeLab) references laboratories(code)
    )engine=innodb;

create table qualifieds (
    num_pass int(5) not null,
    area_assigned int(5),
    lab int(5),
    primary key (num_pass),
    foreign key (num_pass) references employees(num_pass),
    foreign key (area_assigned, lab) references areas (code, codeLab)
    )engine=innodb;

现在我希望得到哪些领域有超过三名合格的员工。具体来说，我想获得区号以及实验室名称，实验室和按地区排序。

我尝试使用此命令来获取区域的代码

select b.code 
from employees e, areas b, qualifieds q 
where e.num_pass=q.num_pass 
and 3 < (select count(b1.code) 
         from areas b1, qualifieds q1, employers e1 
         where e1.num_pass=q1.num_pass
         and q1.area_assigned=b1.code 
         and q1.lab=b1.codeLab);

但我得到的是所有区号的列表，重复次数与员工人数一样多（我有代码1,2,3,4和6名员工的区域，我得到的是序列1， 2,3,4重复6次）。如何获取我需要的信息？

Answer 1

您可以使用GROUP BY和HAVING查找超过3条记录的组...

select areas_assigned, lab, count(num_pass)
    from qualifieds
    group by areas_assigned, lab
    having count(num_pass)>3;

如果您需要对此进行扩展，请添加联接...

select q.areas_assigned, l.name, count(q.num_pass)
    join laboratories l on l.lab = q.lab
    from qualifieds q
    group by q.areas_assigned, l.name
    having count(q.num_pass)>3;

获得哪些领域拥有三名以上合格员工

1 个答案: