MariaDB 10.2

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create table company_m(CompanyID int, CompName varchar(10))
create table client_m(clid int,AttachToCompanyID int, clname varchar(10))
create table companyuser_m(uid int, CompanyID int,uname varchar(10))
insert into company_m(CompanyID,CompName)values(7,'Comp3'),(8,'Comp2'),(9,'Comp1'),(10,'Company 3'),(11,'Company4');
insert into client_m(clid,AttachToCompanyID,clname)values(1,7,'client1'),(2,7,'client2'),(6,8,'client3');
insert into companyuser_m(uid,CompanyID,uname)values(1,7,'user1'),(2,10,'user2');
select company_m.CompanyID,company_m.CompName,count(client_m.AttachToCompanyID) as clientcount 
from company_m
left join client_m on client_m.AttachToCompanyID=company_m.CompanyID
group by company_m.CompanyID 


CompanyID
CompName
clientcount


7
Comp3
2

8
Comp2
1

9
Comp1
0

10
Company 3
0

11
Company4
0


select company_m.CompanyID,company_m.CompName ,count(companyuser_m.CompanyID) as usercount
from company_m
left join companyuser_m on companyuser_m.CompanyID=company_m.CompanyID
group by company_m.CompanyID 


CompanyID
CompName
usercount


7
Comp3
1

8
Comp2
0

9
Comp1
0

10
Company 3
1

11
Company4
0


select company_m.CompanyID,company_m.CompName,
count(client_m.AttachToCompanyID) as clientcount ,
count(companyuser_m.CompanyID) as usercount from company_m
left join client_m on client_m.AttachToCompanyID=company_m.CompanyID
left join companyuser_m on companyuser_m.CompanyID=company_m.CompanyID
group by company_m.CompanyID 


CompanyID
CompName
clientcount
usercount


7
Comp3
2
2

8
Comp2
1
0

9
Comp1
0
0

10
Company 3
0
1

11
Company4
0
0