SELECT dept ,COUNT(1) rcNumber FROM 员工表 
WHERE ( 入职时间 != ''
  OR 入职时间 IS NOT NULL) and DATE_FORMAT(入职时间, '%Y-%m')= '2019-09'
GROUP BY 部门ID
ORDER BY 部门名称

SELECT dept ,COUNT(1) rcNumber FROM 员工表 
WHERE ( 离职时间 != ''
  OR 离职时间 IS NOT NULL) and DATE_FORMAT(入职时间, '%Y-%m')= '2019-09'
GROUP BY 部门ID
ORDER BY 部门名称

SELECT a.dept,a.rcNumber,0 as lcNumber FROM (SELECT dept ,COUNT(1) rcNumber FROM 员工表 
WHERE ( 入职时间 != ''
  OR 入职时间 IS NOT NULL) and DATE_FORMAT(入职时间, '%Y-%m')= '2019-09'
GROUP BY 部门ID
ORDER BY 部门名称) a

SELECT a.dept,a.lcNumber,0 as rcNumber FROM (SELECT dept ,COUNT(1) rcNumber FROM 员工表 
WHERE ( 离职时间 != ''
  OR 离职时间 IS NOT NULL) and DATE_FORMAT(入职时间, '%Y-%m')= '2019-09'
GROUP BY 部门ID
ORDER BY 部门名称) a

SELECT dept ,sum(cm_1) as rcNumber,sum(cm_0) as lcNumber FROM( SELECT c.id,c.dept,SUM(c.lcNumber) as cm_0,c.rcNumber as cm_1 FROM 
(SELECT a.dept,a.rcNumber,0 as lcNumber FROM (SELECT dept ,COUNT(1) rcNumber FROM 员工表 
WHERE ( 入职时间 != ''
  OR 入职时间 IS NOT NULL) and DATE_FORMAT(入职时间, '%Y-%m')= '2019-09'
GROUP BY 部门ID
ORDER BY 部门名称) a) c GROUP BY c.dept
UNION ALL 
SELECT d.id,d.dept,d.lcNumber as cm_0,SUM(d.rcNumber) as cm_1 FROM 
(SELECT a.dept,a.lcNumber,0 as rcNumber FROM (SELECT dept ,COUNT(1) rcNumber FROM 员工表 
WHERE ( 离职时间 != ''
  OR 离职职时间 IS NOT NULL) and DATE_FORMAT(入职时间, '%Y-%m')= '2019-09'
GROUP BY 部门ID
ORDER BY 部门名称) a) d GROUP BY d.dept) t GROUP BY t.dept ORDER BY t.id