SQL(oracle) 取得分組後最大值記錄
|
select * from (select t.*, row_number() over(partition by 分組欄位 order by 排序欄位 desc ) rn from tablename t ) where rn=1 |
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，而此函式計算的值就表示每組內部排序後的順序編號（組內連續的唯一的).
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偽列rownum然後再進行排序，而此函式在包含排序從句後是先排序再計算行號碼．
row_number()和rownum差不多，功能更強一點（可以在各個分組內從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（同樣是在各個分組內）．
dense_rank()l是連續排序，有兩個第二名時仍然跟著第三名。相比之下row_number是沒有重複值的 ．
lag（arg1,arg2,arg3):
arg1是從其他行返回的表示式
arg2是希望檢索的當前行分割槽的偏移量。是一個正的偏移量，時一個往回檢索以前的行的數目。
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。