我如何在postgresql中查詢min，median和max

Question

我寫了一個查詢，其中一列是一個月。從那我必須得到最小月份，最大月份和中間月份。以下是我的查詢。

select ext.employee, 
     pl.fromdate, 
     ext.FULL_INC as full_inc, 
     prevExt.FULL_INC as prevInc, 
     (extract(year from age (pl.fromdate))*12 +extract(month from age (pl.fromdate))) as month, 
     case 
     when prevExt.FULL_INC is not null then (ext.FULL_INC -coalesce(prevExt.FULL_INC,0)) 
     else 0 
     end as difference, 
     (case when prevExt.FULL_INC is not null then (ext.FULL_INC - prevExt.FULL_INC)/prevExt.FULL_INC*100 else 0 end) as percent 
from pl_payroll pl 
    inner join pl_extpayfile ext 
      on pl.cid = ext.payrollid 
     and ext.FULL_INC is not null 
    left outer join pl_extpayfile prevExt 
       on prevExt.employee = ext.employee 
       and prevExt.cid = (select max (cid) from pl_extpayfile 
           where employee = prevExt.employee 
           and payrollid = (
            select max(p.cid) 
            from pl_extpayfile, 
             pl_payroll p 
            where p.cid = payrollid 
            and pl_extpayfile.employee = prevExt.employee 
            and p.fromdate < pl.fromdate 
           )) 
       and coalesce(prevExt.FULL_INC, 0) > 0 
where ext.employee = 17 
and (exists (
    select employee 
    from pl_extpayfile preext 
    where preext.employee = ext.employee 
    and preext.FULL_INC <> ext.FULL_INC 
    and payrollid in (
     select cid 
     from pl_payroll 
     where cid = (
     select max(p.cid) 
     from pl_extpayfile, 
      pl_payroll p 
     where p.cid = payrollid 
     and pl_extpayfile.employee = preext.employee 
     and p.fromdate < pl.fromdate 
    ) 
    ) 
) 
    or not exists (
    select employee 
    from pl_extpayfile fext, 
     pl_payroll p 
    where fext.employee = ext.employee 
    and p.cid = fext.payrollid 
    and p.fromdate < pl.fromdate 
    and fext.FULL_INC > 0 
) 
) 
order by employee, 
     ext.payrollid desc 

如果不可能獲得最大月份和最小月份。

Answer 1

您需要名爲min和max的聚合函數。請參閱PostgreSQL文檔和教程：

• http://www.postgresql.org/docs/current/static/tutorial-agg.html
• http://www.postgresql.org/docs/current/static/functions-aggregate.html

沒有內置在PostgreSQL中位數，但一個已經實施，並促成了維基：

http://wiki.postgresql.org/wiki/Aggregate_Median

它的使用方法與min和相同一旦你已經加載它，。在PL/PgSQL中編寫它會稍微慢一些，但是如果速度很重要的話，甚至可以使用C版本。

UPDATE評論後：

這聽起來像你要顯示的統計總數旁邊的單個結果。你不能用一個簡單的聚合函數來做到這一點，因爲你不能在結果列表中引用不在GROUP BY中的列。

您需要從子查詢中獲取統計信息，或者將聚合用作窗口函數。

由於虛擬數據：

CREATE TABLE dummystats (depname text, empno integer, salary integer); 
INSERT INTO dummystats(depname,empno,salary) VALUES 
('develop',11,5200), 
('develop',7,4200), 
('personell',2,5555), 
('mgmt',1,9999999); 

...並添加the median aggregate from the PG wiki後：

您可以用普通骨料做到這一點：

regress=# SELECT min(salary), max(salary), median(salary) FROM dummystats; 
min | max |   median   
------+---------+---------------------- 
4200 | 9999999 | 5377.5000000000000000 
(1 row) 

但不是這樣的：

regress=# SELECT depname, empno, min(salary), max(salary), median(salary) 
regress-# FROM dummystats; 
ERROR: column "dummystats.depname" must appear in the GROUP BY clause or be used in an aggregate function 

，因爲它在聚合模型中顯示平均數與個別值並無關係。你可以顯示組：

regress=# SELECT depname, min(salary), max(salary), median(salary) 
regress-# FROM dummystats GROUP BY depname; 
    depname | min | max |   median   
-----------+---------+---------+----------------------- 
personell | 5555 | 5555 | 5555.0000000000000000 
develop | 4200 | 5200 | 4700.0000000000000000 
mgmt  | 9999999 | 9999999 | 9999999.000000000000 
(3 rows) 

...但它聽起來像你想要的個人價值觀。爲此，您必須使用PostgreSQL 8.4中的新功能window。

regress=# SELECT depname, empno, 
       min(salary) OVER(), 
       max(salary) OVER(), 
       median(salary) OVER() 
      FROM dummystats; 

    depname | empno | min | max |  median   
-----------+-------+------+---------+----------------------- 
develop | 11 | 4200 | 9999999 | 5377.5000000000000000 
develop |  7 | 4200 | 9999999 | 5377.5000000000000000 
personell |  2 | 4200 | 9999999 | 5377.5000000000000000 
mgmt  |  1 | 4200 | 9999999 | 5377.5000000000000000 
(4 rows) 

參見：

• http://www.postgresql.org/docs/current/static/tutorial-window.html
• http://www.postgresql.org/docs/current/static/functions-window.html

Answer 2

要計算中位數的PostgreSQL，乾脆把50％百分位，無需添加額外的功能或任何東西。

SELECT PERCENTILE_CONT(0.5) WITHIN GROUP(ORDER by X) FROM T;