mysql - 根据字段中的值删除重复行

Question

我有一个非常大的表(几百万条记录)。某些记录具有重复项(基于 FieldA)，唯一的区别是 FiedldB 中的值。我想创建一个查询，该查询将删除基于 FieldA 的所有重复记录，保留 FieldB 中具有最低值的记录。这可能吗？

Answer 1

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函数可以识别重复记录，并方便只删除重复记录，保留原始行。这已经在这里讨论过:Deleting duplicates rows from oracle 。希望这有帮助

但是，由于 DELETE 本身较慢，因此可以在这种情况下(包含数百万条记录)在 INSERT 处实现适当的约束，以避免重复输入。