数据库的左连接查询

表t1,t2

目标：保存t1全部，需要t2中的某些数据

方法：左连接查询

select  t1.*,STATION_NAME 
  from  t1 left join  t2
   on t1.NEW_ID = t2.NEW_ID

3个表的左连接查询

select t1.*,t2.STATION_ID as TID,t3.STATION_NAME
from
( NEW_INSTRUMENTS  t1 left join NEW_STATIONINSTR  t2
on t1.INSTR_ID  = t2.INSTR_ID )
left join NEW_STATIONINFO t3
on t1.NEW_ID  = t3.NEW_ID