mysql问题：字段名前面的a. b. c. d.是什么意思，谁能看懂这段代码

Question

INSERT T_NZLHIS(UPLDDTE,NZLID,NZLNAME,NZLCNT,NZLERR,NZLPER,NZLNPC,NZLRJP,NZLFMDF,NZLMDF,NZLMTIME,NZLMID,LASTCNT,LASTERR,RESULTAVG) 
SELECT a.*,0,0,d.NZLCNT as LASTCNT,d.NZLERR as LASTERR,
(CASE a.NZLCNT-d.NZLCNT 
WHEN 0 THEN d.RESULTAVE ELSE 
(a.NZLERR-d.NZLERR)/(a.NZLCNT-d.NZLCNT) END) as RESULTAVG 
FROM T_NZLBLN a,
(select NZLID,NZLCNT,NZLERR,RESULTAVG from T_NZLHIS b
inner join
(select max(id) as id
from T_NZLHIS
group by NZLID) c on b.id=c.id) d 
where a.NZLID>'' and a.NZLID=d.NZLID;

Answer 1

 表或者子查询的别名
T_NZLBLN a，a就是表T_NZLBLN

(select max(id) as id
from T_NZLHIS
group by NZLID) c
c 就是前面的那个子查询

别的类似