方式1：st_distance_sphere:没用除以1000，所以是以米为单位
SELECT *,st_distance_sphere(point(lng,lat),point(116.3424590000,40.0497810000)) as distance FROM school ORDER BY distance ASC
方式2：st_distance:st_distance 计算的结果单位是度，需要乘111195（地球半径6371000*PI/180）是将值转化为米。
SELECT *,(st_distance(point(lng,lat),point(116.3424590000,40.0497810000))*111195/1000 ) as distance FROM school ORDER BY distance ASC
方式3:
select *,
ROUND(6378.138*2*ASIN(SQRT(POW(SIN(({$lat}*PI()/180-lat*PI()/180)/2),2)+COS({$lat}*PI()/180)*COS(lat*PI()/180)*POW(SIN(({$lng}*PI()/180-lng*PI()/180)/2),2)))*1000) AS distance
from school order by distance asc
