oracle 分組後，取各組的前n條記錄的sql語句

row_number() OVER (PARTITION BY COL1 ORDER BY COL2)

表示根據COL1分組，在分組內部根據 COL2排序，而此函數計算的值就表示每組內部排序後的順序編號（組內連續的惟一的).

　　與rownum的區別在於：使用rownum進行排序的時候是先對結果集加入僞列rownum而後再進行排序，而此函數在包含排序從句後是先排序再計算行號碼．

　　row_number()和rownum差很少，功能更強一點（能夠在各個分組內從1開時排序）．

　　rank()是跳躍排序，有兩個第二名時接下來就是第四名（一樣是在各個分組內）．

　　dense_rank()l是連續排序，有兩個第二名時仍然跟着第三名。相比之下row_number是沒有重複值的 ．

　　lag（arg1,arg2,arg3): 
                    arg1是從其餘行返回的表達式 
                    arg2是但願檢索的當前行分區的偏移量。是一個正的偏移量，時一個往回檢索之前的行的數目。 
                    arg3是在arg2表示的數目超出了分組的範圍時返回的值。

看幾個SQL語句:

語句一:

select row_number() over(order by sale/cnt desc) as sort, sale/cnt 
from (
select -60 as sale,3 as cnt from dual union 
select 24 as sale,6 as cnt from dual union 
select 50 as sale,5 as cnt from dual union 
select -20 as sale,2 as cnt from dual union 
select 40 as sale,8 as cnt from dual
);

執行結果:

          SORT       SALE/CNT
---------- ----------
             1             10
             2              5
             3              4
             4            -10
             5            -20

 

語句二:查詢員工的工資,按部門排序

select ename,sal,row_number() over (partition by deptno order by sal desc) as sal_order from scott.emp;

執行結果:

ENAME                           SAL      SAL_ORDER
-------------------- ---------- ----------
KING                           5000              1
CLARK                          2450              2
MILLER                         1300              3
SCOTT                          3000              1
FORD                           3000              2
JONES                          2975              3
ADAMS                          1100              4
SMITH                           800              5
BLAKE                          2850              1
ALLEN                          1600              2
TURNER                         1500              3
WARD                           1250              4
MARTIN                         1250              5
JAMES                           950              6

已選擇14行。

語句三:查詢每一個部門的最高工資

select deptno,ename,sal from 
     (select deptno,ename,sal,row_number() over (partition by deptno order by sal desc) as sal_order 
         from scott.emp) where sal_order <2;

執行結果:

       DEPTNO ENAME                          SAL
---------- -------------------- ----------
           10 KING                          5000
           20 SCOTT                         3000
           30 BLAKE                         2850

已選擇3行。

語句四：

select deptno,sal,rank() over (partition by deptno order by sal) as rank_order from scott.emp order by deptno;

執行結果:

     DEPTNO         SAL RANK_ORDER
---------- ---------- ----------
         10        1300           1
         10        2450           2
         10        5000           3
         20         800           1
         20        1100           2
         20        2975           3
         20        3000           4
         20        3000           4
         30         950           1
         30        1250           2
         30        1250           2
         30        1500           4
         30        1600           5
         30        2850           6

已選擇14行。

語句五：

select deptno,sal,dense_rank() over(partition by deptno order by sal) as dense_rank_order from scott.emp order by deptn;

執行結果：

     DEPTNO         SAL DENSE_RANK_ORDER
---------- ---------- ----------------
         10        1300                 1
         10        2450                 2
         10        5000                 3
         20         800                 1
         20        1100                 2
         20        2975                 3
         20        3000                 4
         20        3000                 4
         30         950                 1
         30        1250                 2
         30        1250                 2
         30        1500                 3
         30        1600                 4
         30        2850                 5

已選擇14行。

語句六：

select deptno,ename,sal,lag(ename,1,null) over(partition by deptno order by ename) as lag_ from scott.emp order by deptno;

執行結果：

     DEPTNO ENAME                        SAL LAG_
---------- -------------------- ---------- --------------------
         10 CLARK                       2450
         10 KING                        5000 CLARK
         10 MILLER                      1300 KING
         20 ADAMS                       1100
         20 FORD                        3000 ADAMS
         20 JONES                       2975 FORD
         20 SCOTT                       3000 JONES
         20 SMITH                        800 SCOTT
         30 ALLEN                       1600
         30 BLAKE                       2850 ALLEN
         30 JAMES                        950 BLAKE
         30 MARTIN                      1250 JAMES
         30 TURNER                      1500 MARTIN
         30 WARD                        1250 TURNER

已選擇14行。

============================================

轉自：http://wing123.iteye.com/blog/718751

分組時不只想取每組的第一條記錄，還想取每組共有幾條記錄

SELECT *
FROM (
SELECT ROW_NUMBER() OVER(PARTITION BY PK_FINANCE_ACCOUNT ORDER BY amassdate DESC) LEV, count(0) over(partition by PK_FINANCE_ACCOUNT) cnt, hx_amass_liquid.*
 FROM hx_amass_liquid
)
 WHERE LEV = 1
 ORDER BY PK_FINANCE_ACCOUNT DESC