select中case when的使用_MySQL

SELECT   grade, COUNT (CASE WHEN sex = 1 THEN 1      /*sex 1为男生，2位女生*/                                            ELSE NULL                                            END) 男生数,                            COUNT (CASE WHEN sex = 2 THEN 1                                            ELSE NULL                                            END) 女生数    FROM students GROUP BY grade

pat_visit.admission_date_time的日期格式为YYYY-MM-DD HH24:MI,求患者的年龄。create orreplaceview yw_viewas( selectcasewhen trunc(a.admission_date_time-b.date_of_birth)<30 and floor(a.admission_date_time-b.date_of_birth)>=1 then  round(a.admission_date_time-b.date_of_birth)||'天'  whenround(a.admission_date_time-b.date_of_birth)>30 andround(a.admission_date_time-b.date_of_birth)<365 then  round((a.admission_date_time-b.date_of_birth)/30)||'月' whenfloor(a.admission_date_time-b.date_of_birth)>'365'then  round((a.admission_date_time-b.date_of_birth)/365)||'岁'  whenceil(a.admission_date_time-b.date_of_birth)<='1'then  round((a.admission_date_time-b.date_of_birth)*24*60)||'分钟'  end  as年龄,a.patient_idfrom pat_visit a,pat_master_index bwhere a.patient_id=b.patient_id);