Значения COUNT в разных столбцах одной таблицы SQL

Question

Например, у моей таблицы есть данные - 10 добровольцев. Есть два типа добровольцев - студенты и сотрудники. Как можно вставить несколько столбцов в одной таблице с этой точкой зрения:

COUNT (Volunteer_id), COUNT (Volunteer_id) WHERE Volunteer_type = 'Студент', COUNT (Volunteer_id WHERE Volunteer_type = 'Персонал'

SELECT COUNT(Volunteer_id) AS "TOTAL VOLUNTEERS" 
from volunteer 
UNION 
SELECT COUNT(Volunteer_id) AS "TOTAL VOLUNTEERS" 
from volunteer 
WHERE Volunteer_type = 'Staff' 
UNION 
SELECT COUNT(Volunteer_id) AS "TOTAL VOLUNTEERS" 
from volunteer 
WHERE Volunteer_type = 'Student' 

Эти утверждения представляются в виде строк на данный момент, но я хочу, чтобы сделать их столбцы,

Answer 1

Использование условной агрегации:

SELECT 
    COUNT(*) AS "Total Volunteers", 
    COUNT(CASE WHEN Volunteer_tpye = 'Staff' THEN 1 END) AS "Staff Volunteers", 
    COUNT(CASE WHEN Volunteer_tpye = 'Student' THEN 1 END) AS Student 
FROM volunteers 

Answer 2

DECLARE @volunteer TABLE 
(Volunteer_id int 
,Volunteer_type varchar(50)) 

INSERT INTO @volunteer VALUES (1,'Staff') 
INSERT INTO @volunteer VALUES (1,'Staff') 
INSERT INTO @volunteer VALUES (1,'Staff') 

INSERT INTO @volunteer VALUES (1,'Student') 
INSERT INTO @volunteer VALUES (1,'Student') 
INSERT INTO @volunteer VALUES (1,'Student') 


SELECT COUNT(Volunteer_id) AS "TOTAL VOLUNTEERS" 
     ,(
      SELECT COUNT(tb1.Volunteer_id) AS "TOTAL VOLUNTEERS" 
      FROM @volunteer AS tb1 
      WHERE tb1.Volunteer_id = tb.Volunteer_id 
      AND tb1.Volunteer_type = 'Staff' 


     ) 
     ,(
      SELECT COUNT(volunteer_Student.Volunteer_id) AS "TOTAL VOLUNTEERS" 
      FROM @volunteer AS volunteer_Student 
      WHERE volunteer_Student.Volunteer_id = tb.Volunteer_id 
      AND volunteer_Student.Volunteer_type = 'Student' 
     ) 
FROM @volunteer tb 
GROUP BY tb.Volunteer_id 

Answer 3

Попробуйте это:

select 
    sum(t.x_all) cnt_all, 
    sum(t.x_staff) cnt_staff, 
    sum(t.x_student) cnt_student 
from (
    select 
    1 x_all, 
    case when v.volunteer_type = 'Staff' then 1 else 0 end x_staff, 
    case when v.volunteer_type = 'Student' then 1 else 0 end x_student 
    from volunteer v 
) t