用postgresql特性简化group by 后取每组前n条记录的方法

需求:

求一段时间内各生产线前3名坏机的原因及坏机数.

step 1, 从将原始数据中抽取品质数据放于t96临时表

create temp table t96 
on commit drop 
as 
select * from t96_pd_log where recseq between "791G0" and "792G0" and f96_op="PD-QC"  ;

可以看下这个t96中对坏机原因的统计情况:

select f96_line,f96_result, sum(f96_qty) as qty from t96 group by f96_line,f96_result order by f96_line,f96_result,qty desc

step2, 一条sql就能得到需求的结果, 其中用了with这个短句,类似于临时表或子查询的意思了,但如果用了with定义一个子集,在接着Select里面可以多处引用.  下面SQL这个A子查询每条生产线用不良数倒序排,并每记录给一个行号 ...

with udc_qty_byline as 
(select f96_line,f96_result, sum(f96_qty) as qty from t96 group by f96_line,f96_result )
SELECT f96_line,f96_result,qty FROM 
(
    SELECT *, ROW_NUMBER() OVER (PARTITION BY 
             f96_line order by qty DESC) AS Row_ID FROM udc_qty_byline
) AS A
WHERE Row_ID <=3 ORDER BY f96_line

  Postgresql is powerful.