根据字段中的值删除重复的行

Question

我有一个非常大的表（数百万条记录）。有些记录有重复（基于FieldA），唯一的区别是FiedldB中的值。我想创建一个查询，它将删除基于FieldA的所有重复记录，并保留FieldB中值最低的记录。这可能吗？

Answer 1

似乎相当简单的提取这些值：

select distinct a, 
     min(b) b 
from t 
group by a; 

小提琴例如：http://sqlfiddle.com/#!9/bc4c9/3

您应该能够从这种适应去除方法。

Answer 2

CREATE TABLE TABLE1 
    (
    FieldA VARCHAR2(30), 
    FieldB VARCHAR2(30), 
    FieldC VARCHAR2(30) 
); 

INSERT INTO TABLE1 VALUES 
    ('DUMMYDATA-A1','DUMMYDATA-B1','DUMMYDATA-C1' 
); 
INSERT INTO TABLE1 VALUES 
    ('DUMMYDATA-A1','DUMMYDATA-B4','DUMMYDATA-C1' 
); 
INSERT INTO TABLE1 VALUES 
    ('DUMMYDATA-A1','DUMMYDATA-B3','DUMMYDATA-C1' 
); 
INSERT INTO TABLE1 VALUES 
    ('DUMMYDATA-A1','DUMMYDATA-B2','DUMMYDATA-C1' 
); 
COMMIT; 

SELECT FieldA, 
    FieldB, 
    FieldC, 
    RANK() OVER(PARTITION BY FieldA ORDER BY FieldB ASC) AS COLUMN_ALIAS 
FROM TABLE1; --IDENTIFIES DUPLICATES BASED ON RANK VALUE 

---PERFORM DELETE 
DELETE 
FROM TABLE1 
WHERE ROWID IN 
    (SELECT ROWID 
    FROM 
    (SELECT ROWID, 
     RANK() OVER(PARTITION BY FieldA ORDER BY FieldB ASC) AS COLUMN_ALIAS 
    FROM TABLE1 
    ) 
    WHERE COLUMN_ALIAS>1 
); 

COMMIT; 

SELECT * FROM TABLE1; -- CONTAINS A SINGLE RECORD 

RANK函数可以识别重复的记录，并且便于删除重复的记录，保留原始行。这已经在这里讨论：Deleting duplicates rows from oracle。希望这可以帮助

但是，由于DELETE本身速度较慢，因此可以在INSERT中执行适当的约束（包含数百万条记录）以避免重复输入。