找到有最多员工的项目（多个最大值）

LeetCode 1076题

project
image.png

1、先找到最大的数是多少，再分组筛选数量=最大的项目id

SELECT project_id
FROM Project
GROUP BY project_id
HAVING COUNT(*) = (
           SELECT COUNT(*)     #找到最大的数字
           FROM Project
           GROUP BY project_id
           ORDER BY COUNT(*) DESC
           LIMIT 1
            )

2、筛选条件≥ALL（子查询）

SELECT project_id
FROM Project
GROUP BY project_id
HAVING COUNT(*) >= ALL (
        SELECT COUNT(*)    
        FROM Project
        GROUP BY project_id
        )

3、窗口函数

（注意group by和partition by用法）

SELECT project_id
FROM (
    SELECT project_id, rank() OVER(ORDER BY COUNT(employee_id) DESC) rk
    FROM Project
    GROUP BY project_id
) t
WHERE t.rk=1