通過運行查詢，以防止重複記錄

Question

避免在表中重複記錄Updatting表

我有一個表像一個下面

CREATE TABLE models(model_id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, 
         model_no varchar(255));        

INSERT INTO models(model_no) 
       VALUES('M24'), 
         ('M22'), 
         ('M25'), 
         ('M24'), 
         ('M24'),    
         ('M35'), 
         ('M15'), 
         ('M18'), 
         ('M25'), 
         ('M15'), 
         ('M15'), 
         ('M17'), 
         ('M19'), 
         ('M29'), 
         ('M29'), 
         ('M12'), 
         ('M12'), 
         ('M13'), 
         ('M29'); 

我想通過運行一個DELETE刪除此表中的重複模式或者，這樣的模式將只出現一次，如下

1 M24 
2 M22 
3 M25 
6 M35 
7 M15 
8 M18 
12 M17 
13 M19 
14 M29 
16 M12 
18 M13 

Answer 1

delete m from models m 
join models m2 on m2.model_no = m.model_no and m.model_id > m2.model_id 

Answer 2

要刪除已存在於表中可以使用重複更新查詢：

CREATE TEMPORARY TABLE tmp (ModelID INT NOT NULL); 

INSERT tmp (ModelID) 
SELECT t1.Model_ID 
FROM Models t1 
WHERE EXISTS 
     ( SELECT 1 
      FROM Models t2 
      WHERE t1.Model_No = t2.Model_No 
      AND  t2.Model_ID < t1.Model_ID 
     ); 

DELETE 
FROM Models 
WHERE Model_ID IN (SELECT ModelID FROM tmp); 

SQL Fiddle

然後前進，你應該添加一個唯一索引，以阻止進一步的重複。

CREATE UNIQUE INDEX UQ_Models ON Models (Model_No); 

Answer 3

嘗試使用臨時表。

create temporary table tmpTable (model_id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, 
        model_no varchar(255)); 



insert tmpTable 
    (model_id) 
select model_id 
from models m 
where exists 
    (
    select * 
    from models m2 
    where m2.model_no = m.model_no 

      and m2.model_id> m.model_id 
    ); 

delete 
from models 
where model_id in (select model_id from tmpTable); 

Answer 4

試試這個：

ALTER IGNORE TABLE `models` 
ADD UNIQUE INDEX (`model_no`);