sql - COUNT 一个表的不同列中的值 SQL

Question

例如，我的表有数据 - 10 名志愿者。 志愿者有两种类型——学生和工作人员。 如何使用此 View 在一个表中插入几列:

COUNT(Volunteer_id), COUNT(Volunteer_id) WHERE Volunteer_type = '学生', COUNT(Volunteer_id WHERE Volunteer_type = '员工'

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'

这些语句目前表示为行，但我想让它们成为列

Answer 1

使用条件聚合:

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