不满足条件的查询计数为 1

Question

i have a two table i want to know the number of person who are all assigned to project in each sector

CREATE TABLE first1( a int,projectname varchar(20));
INSERT INTO first1 VALUES
(1001,'crm'),
(1002,'iic'),
(1003,'abc'),
(1004,'sifty bank');

CREATE TABLE diff(b int,name varchar(20),p_id int );
INSERT INTO diff VALUES
(101,'priya',1001),
(102,'divya',1002),
(103,'sidhu',null),
(104,'shiva',null),
(105,'surya',1002);

Query:

select first1.projectname,count(*) from first1  left join diff on first1.a=diff.p_id group by 
first1.projectname;

The output of this code is:

abc|1
crm|1
iic|2
sifty bank|1

The expected output is:

abc|0
crm|1
iic|2
sifty bank|0

Answer 1

The problem is count(*) ; it counts how many rows there are in each group - A project without any person assigned still counts as 1. Instead, you need to count() something from the left table, so null values are not taken into account:

select f.projectname, count(d.p_id) as cnt_diff
from first1 f
left join diff d on f.a = d.p_id 
group by f.projectname;

Note that you can get the same result with a subquery:

select f.projectname, 
    (select count(*) from diff d where d.p_id = f.a)  as cnt_diff
from first1 f