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In questa sezione impareremo diversi modi per eliminare le righe duplicate in MySQL e Oracle . Se la SQL La tabella contiene righe duplicate, quindi dobbiamo rimuovere le righe duplicate.

Preparazione dei dati del campione

Lo script crea la tabella denominata contatti .

 DROP TABLE IF EXISTS contacts; CREATE TABLE contacts ( id INT PRIMARY KEY AUTO_INCREMENT, first_name VARCHAR(30) NOT NULL, last_name VARCHAR(25) NOT NULL, email VARCHAR(210) NOT NULL, age VARCHAR(22) NOT NULL ); 

Nella tabella sopra abbiamo inserito i seguenti dati.

tipi di dati successivi

 INSERT INTO contacts (first_name,last_name,email,age) VALUES ('Kavin','Peterson','[email protected]','21'), ('Nick','Jonas','[email protected]','18'), ('Peter','Heaven','[email protected]','23'), ('Michal','Jackson','[email protected]','22'), ('Sean','Bean','[email protected]','23'), ('Tom ','Baker','[email protected]','20'), ('Ben','Barnes','[email protected]','17'), ('Mischa ','Barton','[email protected]','18'), ('Sean','Bean','[email protected]','16'), ('Eliza','Bennett','[email protected]','25'), ('Michal','Krane','[email protected]','25'), ('Peter','Heaven','[email protected]','20'), ('Brian','Blessed','[email protected]','20'); ('Kavin','Peterson','[email protected]','30'), 

Eseguiamo lo script per ricreare i dati di test dopo aver eseguito a ELIMINARE dichiarazione .

La query restituisce i dati dalla tabella dei contatti:

 SELECT * FROM contacts ORDER BY email; 

La seguente query SQL restituisce le email duplicate dalla tabella dei contatti:

 SELECT email, COUNT(email) FROM contacts GROUP BY email HAVING COUNT (email) > 1; 

Abbiamo tre righe con duplicare e-mail.

(A) Elimina le righe duplicate con l'istruzione DELETE JOIN

 DELETE t1 FROM contacts t1 INNERJOIN contacts t2 WHERE t1.id <t2.id and t1.email="t2.email;" < pre> <p> <strong>Output:</strong> </p> <pre> Query OK, three rows affected (0.10 sec) </pre> <p>Three rows had been deleted. We execute the query, given below to finds the <strong>duplicate emails</strong> from the table.</p> <pre> SELECT email, COUNT (email) FROM contacts GROUP BY email HAVING COUNT (email) &gt; 1; </pre> <p>The query returns the empty set. To verify the data from the contacts table, execute the following SQL query:</p> <pre> SELECT * FROM contacts; </pre> <br> <table class="table"> <tr> <td>id</td> <td>first_name</td> <td>last_name</td> <td>Email</td> <td>age</td> </tr> <tr> <td>7</td> <td>Ben</td> <td>Barnes</td> <td> [email protected] </td> <td>21</td> </tr> <tr> <td>13</td> <td>Brian</td> <td>Blessed</td> <td> [email protected] </td> <td>18</td> </tr> <tr> <td>10</td> <td>Eliza</td> <td>Bennett</td> <td> [email protected] </td> <td>23</td> </tr> <tr> <td>1</td> <td>Kavin</td> <td>Peterson</td> <td> [email protected] </td> <td>22</td> </tr> <tr> <td>8</td> <td>Mischa</td> <td>Barton</td> <td> [email protected] </td> <td>20</td> </tr> <tr> <td>11</td> <td>Micha</td> <td>Krane</td> <td> [email protected] </td> <td>17</td> </tr> <tr> <td>4</td> <td>Michal</td> <td>Jackson</td> <td> [email protected] </td> <td>18</td> </tr> <tr> <td>2</td> <td>Nick</td> <td>Jonas</td> <td> [email protected] </td> <td>16</td> </tr> <tr> <td>3</td> <td>Peter</td> <td>Heaven</td> <td> [email protected] </td> <td>25</td> </tr> <tr> <td>5</td> <td>Sean</td> <td>Bean</td> <td> [email protected] </td> <td>20</td> </tr> <tr> <td>6</td> <td>Tom</td> <td>Baker</td> <td> [email protected] </td> <td>30</td> </tr> </table> <p>The rows <strong>id&apos;s 9, 12, and 14</strong> have been deleted. We use the below statement to delete the duplicate rows:</p> <p>Execute the script for <strong>creating</strong> the contact.</p> <pre> DELETE c1 FROM contacts c1 INNERJ OIN contacts c2 WHERE c1.id &gt; c2.id AND c1.email = c2.email; </pre> <br> <table class="table"> <tr> <td>id</td> <td>first_name</td> <td>last_name</td> <td>email</td> <td>age</td> </tr> <tr> <td>1</td> <td>Ben</td> <td>Barnes</td> <td> [email protected] </td> <td>21</td> </tr> <tr> <td>2</td> <td> <strong>Kavin</strong> </td> <td> <strong>Peterson</strong></td> <td> <strong> [email protected] </strong> </td> <td> <strong>22</strong> </td> </tr> <tr> <td>3</td> <td>Brian</td> <td>Blessed</td> <td> [email protected] </td> <td>18</td> </tr> <tr> <td>4</td> <td>Nick</td> <td>Jonas</td> <td> [email protected] </td> <td>16</td> </tr> <tr> <td>5</td> <td>Michal</td> <td>Krane</td> <td> [email protected] </td> <td>17</td> </tr> <tr> <td>6</td> <td>Eliza</td> <td>Bennett</td> <td> [email protected] </td> <td>23</td> </tr> <tr> <td>7</td> <td>Michal</td> <td>Jackson</td> <td> [email protected] </td> <td>18</td> </tr> <tr> <td>8</td> <td> <strong>Sean</strong> </td> <td> <strong>Bean</strong> </td> <td> <strong> [email protected] </strong> </td> <td> <strong>20</strong> </td> </tr> <tr> <td>9</td> <td>Mischa</td> <td>Barton</td> <td> [email protected] </td> <td>20</td> </tr> <tr> <td>10</td> <td> <strong>Peter</strong> </td> <td> <strong>Heaven</strong> </td> <td> <strong> [email protected] </strong> </td> <td> <strong>25</strong> </td> </tr> <tr> <td>11</td> <td>Tom</td> <td>Baker</td> <td> [email protected] </td> <td>30</td> </tr> </table> <h2>(B) Delete duplicate rows using an intermediate table</h2> <p>To delete a duplicate row by using the intermediate table, follow the steps given below:</p> <p> <strong>Step 1</strong> . Create a new table <strong>structure</strong> , same as the real table:</p> <pre> CREATE TABLE source_copy LIKE source; </pre> <p> <strong>Step 2</strong> . Insert the distinct rows from the original schedule of the database:</p> <pre> INSERT INTO source_copy SELECT * FROM source GROUP BY col; </pre> <p> <strong>Step 3</strong> . Drop the original table and rename the immediate table to the original one.</p> <pre> DROP TABLE source; ALTER TABLE source_copy RENAME TO source; </pre> <p>For example, the following statements delete the <strong>rows</strong> with <strong>duplicate</strong> emails from the contacts table:</p> <pre> -- step 1 CREATE TABLE contacts_temp LIKE contacts; -- step 2 INSERT INTO contacts_temp SELECT * FROM contacts GROUP BY email; -- step 3 DROP TABLE contacts; ALTER TABLE contacts_temp RENAME TO contacts; </pre> <h2>(C) Delete duplicate rows using the ROW_NUMBER() Function</h2> <h4>Note: The ROW_NUMBER() function has been supported since MySQL version 8.02, so we should check our MySQL version before using the function.</h4> <p>The following statement uses the <strong>ROW_NUMBER ()</strong> to assign a sequential integer to every row. If the email is duplicate, the row will higher than one.</p> <pre> SELECT id, email, ROW_NUMBER() OVER (PARTITION BY email ORDER BY email ) AS row_num FROM contacts; </pre> <p>The following SQL query returns <strong>id list</strong> of the duplicate rows:</p> <pre> SELECT id FROM (SELECT id, ROW_NUMBER() OVER ( PARTITION BY email ORDER BY email) AS row_num FROM contacts ) t WHERE row_num&gt; 1; </pre> <p> <strong>Output:</strong> </p> <table class="table"> <tr> <td>id</td> </tr> <tr> <td>9</td> </tr> <tr> <td>12</td> </tr> <tr> <td>14</td> </tr> </table> <h2>Delete Duplicate Records in Oracle</h2> <p>When we found the duplicate records in the table, we had to delete the unwanted copies to keep our data clean and unique. If a table has duplicate rows, we can delete it by using the <strong>DELETE</strong> statement.</p> <p>In the case, we have a column, which is not the part of <strong>group</strong> used to <strong>evaluate</strong> the <strong>duplicate</strong> records in the table.</p> <p>Consider the table given below:</p> <table class="table"> <tr> <td>VEGETABLE_ID</td> <td>VEGETABLE_NAME</td> <td>COLOR</td> </tr> <tr> <td>01</td> <td>Potato</td> <td>Brown</td> </tr> <tr> <td>02</td> <td>Potato</td> <td>Brown</td> </tr> <tr> <td>03</td> <td>Onion</td> <td>Red</td> </tr> <tr> <td>04</td> <td>Onion</td> <td>Red</td> </tr> <tr> <td>05</td> <td>Onion</td> <td>Red</td> </tr> <tr> <td>06</td> <td>Pumpkin</td> <td>Green</td> </tr> <tr> <td>07</td> <td>Pumpkin</td> <td>Yellow</td> </tr> </table> <br> <pre> -- create the vegetable table CREATE TABLE vegetables ( VEGETABLE_ID NUMBER generated BY DEFAULT AS ID ENTITY, VEGETABLE_NAME VARCHAR2(100), color VARCHAR2(20), PRIMARY KEY (VEGETABLE_ID) ); </pre> <br> <pre> -- insert sample rows INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES(&apos;Potato&apos;,&apos;Brown&apos;); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES(&apos;Potato&apos;,&apos;Brown&apos;); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES(&apos;Onion&apos;,&apos;Red&apos;); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES(&apos;Onion&apos;,&apos;Red&apos;); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES(&apos;Onion&apos;,&apos;Red&apos;); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES(&apos;Pumpkin&apos;,&apos;Green&apos;); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES(&apos;Pumpkin&apos;,&apos;Yellow&apos;); </pre> <br> <pre> -- query data from the vegetable table SELECT * FROM vegetables; </pre> <p>Suppose, we want to keep the row with the highest <strong>VEGETABLE_ID</strong> and delete all other copies.</p> <pre> SELECT MAX (VEGETABLE_ID) FROM vegetables GROUP BY VEGETABLE_NAME, color ORDER BY MAX(VEGETABLE_ID); </pre> <br> <table class="table"> <tr> <td>MAX(VEGETABLE_ID)</td> </tr> <tr> <td>2</td> </tr> <tr> <td>5</td> </tr> <tr> <td>6</td> </tr> <tr> <td>7</td> </tr> </table> <p>We use the <strong>DELETE</strong> statement to delete the rows whose values in the <strong>VEGETABLE_ID COLUMN</strong> are not the <strong>highest</strong> .</p> <pre> DELETE FROM vegetables WHERE VEGETABLE_IDNOTIN ( SELECT MAX(VEGETABLE_ID) FROM vegetables GROUP BY VEGETABLE_NAME, color ); </pre> <p>Three rows have been deleted.</p> <pre> SELECT *FROM vegetables; </pre> <br> <table class="table"> <tr> <td>VEGETABLE_ID</td> <td>VEGETABLE_NAME</td> <td>COLOR</td> </tr> <tr> <td> <strong>02</strong> </td> <td>Potato</td> <td>Brown</td> </tr> <tr> <td> <strong>05</strong> </td> <td>Onion</td> <td>Red</td> </tr> <tr> <td> <strong>06</strong> </td> <td>Pumpkin</td> <td>Green</td> </tr> <tr> <td> <strong>07</strong> </td> <td><pumpkin td> <td>Yellow</td> </pumpkin></td></tr> </table> <p>If we want to keep the row with the lowest id, use the <strong>MIN()</strong> function instead of the <strong>MAX()</strong> function.</p> <pre> DELETE FROM vegetables WHERE VEGETABLE_IDNOTIN ( SELECT MIN(VEGETABLE_ID) FROM vegetables GROUP BY VEGETABLE_NAME, color ); </pre> <p>The above method works if we have a column that is not part of the group for evaluating duplicate. If all values in the columns have copies, then we cannot use the <strong>VEGETABLE_ID</strong> column.</p> <p>Let&apos;s drop and create the <strong>vegetable</strong> table with a new structure.</p> <pre> DROP TABLE vegetables; CREATE TABLE vegetables ( VEGETABLE_ID NUMBER, VEGETABLE_NAME VARCHAR2(100), Color VARCHAR2(20) ); </pre> <br> <pre> INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(1,&apos;Potato&apos;,&apos;Brown&apos;); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(1, &apos;Potato&apos;,&apos;Brown&apos;); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color)VALUES(2,&apos;Onion&apos;,&apos;Red&apos;); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color)VALUES(2,&apos;Onion&apos;,&apos;Red&apos;); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(2,&apos;Onion&apos;,&apos;Red&apos;); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(3,&apos;Pumpkin&apos;,&apos;Green&apos;); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(&apos;4,Pumpkin&apos;,&apos;Yellow&apos;); SELECT * FROM vegetables; </pre> <br> <table class="table"> <tr> <td>VEGETABLE_ID</td> <td>VEGETABLE_NAME</td> <td>COLOR</td> </tr> <tr> <td>01</td> <td>Potato</td> <td>Brown</td> </tr> <tr> <td>01</td> <td>Potato</td> <td>Brown</td> </tr> <tr> <td>02</td> <td>Onion</td> <td>Red</td> </tr> <tr> <td>02</td> <td>Onion</td> <td>Red</td> </tr> <tr> <td>02</td> <td>Onion</td> <td>Red</td> </tr> <tr> <td>03</td> <td>Pumpkin</td> <td>Green</td> </tr> <tr> <td>04</td> <td>Pumpkin</td> <td>Yellow</td> </tr> </table> <p>In the vegetable table, the values in all columns <strong>VEGETABLE_ID, VEGETABLE_NAME</strong> , and color have been copied.</p> <p>We can use the <strong>rowid</strong> , a locator that specifies where Oracle stores the row. Because the <strong>rowid</strong> is unique so that we can use it to remove the duplicates rows.</p> <pre> DELETE FROM Vegetables WHERE rowed NOT IN ( SELECT MIN(rowid) FROM vegetables GROUP BY VEGETABLE_ID, VEGETABLE_NAME, color ); </pre> <p>The query verifies the deletion operation:</p> <pre> SELECT * FROM vegetables; </pre> <br> <table class="table"> <tr> <td>VEGETABLE_ID</td> <td>VEGETABLE_NAME</td> <td>COLOR</td> </tr> <tr> <td>01</td> <td>Potato</td> <td>Brown</td> </tr> <tr> <td>02</td> <td>Onion</td> <td>Red</td> </tr> <tr> <td>03</td> <td>Pumpkin</td> <td>Green</td> </tr> <tr> <td>04</td> <td>Pumpkin</td> <td>Yellow</td> </tr> </table> <hr></t2.id>

Tre righe erano state cancellate. Eseguiamo la query indicata di seguito per trovare il file email duplicate dal tavolo.

 SELECT email, COUNT (email) FROM contacts GROUP BY email HAVING COUNT (email) > 1; 

La query restituisce il set vuoto. Per verificare i dati dalla tabella dei contatti, eseguire la seguente query SQL:

 SELECT * FROM contacts; 

Le righe gli ID sono 9, 12 e 14 sono stati cancellati. Usiamo la seguente istruzione per eliminare le righe duplicate:

Esegui lo script per creando il contatto.

 DELETE c1 FROM contacts c1 INNERJ OIN contacts c2 WHERE c1.id > c2.id AND c1.email = c2.email; 

(B) Elimina le righe duplicate utilizzando una tabella intermedia

Per eliminare una riga duplicata utilizzando la tabella intermedia, seguire i passaggi indicati di seguito:

Passo 1 . Crea una nuova tabella struttura , uguale alla tabella reale:

l'età di salman khan

 CREATE TABLE source_copy LIKE source; 

Passo 2 . Inserisci le righe distinte dalla pianificazione originale del database:

 INSERT INTO source_copy SELECT * FROM source GROUP BY col; 

Passaggio 3 . Rilascia la tabella originale e rinomina la tabella immediata con quella originale.

 DROP TABLE source; ALTER TABLE source_copy RENAME TO source; 

Ad esempio, le seguenti istruzioni eliminano il file righe con duplicare email dalla tabella dei contatti:

 -- step 1 CREATE TABLE contacts_temp LIKE contacts; -- step 2 INSERT INTO contacts_temp SELECT * FROM contacts GROUP BY email; -- step 3 DROP TABLE contacts; ALTER TABLE contacts_temp RENAME TO contacts; 

(C) Elimina le righe duplicate utilizzando la funzione ROW_NUMBER()

Nota: la funzione ROW_NUMBER() è supportata dalla versione MySQL 8.02, quindi dovremmo controllare la nostra versione MySQL prima di utilizzare la funzione.

La seguente istruzione utilizza il file NUMERO_RIGA () per assegnare un numero intero sequenziale a ogni riga. Se l'e-mail è duplicata, la riga sarà superiore a uno.

 SELECT id, email, ROW_NUMBER() OVER (PARTITION BY email ORDER BY email ) AS row_num FROM contacts; 

Viene restituita la seguente query SQL elenco ID delle righe duplicate:

 SELECT id FROM (SELECT id, ROW_NUMBER() OVER ( PARTITION BY email ORDER BY email) AS row_num FROM contacts ) t WHERE row_num> 1; 

Produzione:

Elimina i record duplicati in Oracle

Quando abbiamo trovato i record duplicati nella tabella, abbiamo dovuto eliminare le copie indesiderate per mantenere i nostri dati puliti e unici. Se una tabella ha righe duplicate, possiamo eliminarla utilizzando il comando ELIMINARE dichiarazione.

Nel caso, abbiamo una colonna, che non fa parte di gruppo abituato a valutare IL duplicare record nella tabella.

Consideriamo la tabella riportata di seguito:

 -- create the vegetable table CREATE TABLE vegetables ( VEGETABLE_ID NUMBER generated BY DEFAULT AS ID ENTITY, VEGETABLE_NAME VARCHAR2(100), color VARCHAR2(20), PRIMARY KEY (VEGETABLE_ID) ); 

 -- insert sample rows INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Potato','Brown'); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Potato','Brown'); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Onion','Red'); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Onion','Red'); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Onion','Red'); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Pumpkin','Green'); INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Pumpkin','Yellow'); 

 -- query data from the vegetable table SELECT * FROM vegetables; 

Supponiamo di voler mantenere la riga con il valore più alto ID_VEGETABILE ed eliminare tutte le altre copie.

 SELECT MAX (VEGETABLE_ID) FROM vegetables GROUP BY VEGETABLE_NAME, color ORDER BY MAX(VEGETABLE_ID); 

Noi usiamo il ELIMINARE istruzione per eliminare le righe i cui valori nel file COLONNA VEGETABLE_ID non sono i più alto .

 DELETE FROM vegetables WHERE VEGETABLE_IDNOTIN ( SELECT MAX(VEGETABLE_ID) FROM vegetables GROUP BY VEGETABLE_NAME, color ); 

Sono state cancellate tre righe.

 SELECT *FROM vegetables; 

Se vogliamo mantenere la riga con l'id più basso, usiamo il file MIN() funzione al posto di MASSIMO() funzione.

 DELETE FROM vegetables WHERE VEGETABLE_IDNOTIN ( SELECT MIN(VEGETABLE_ID) FROM vegetables GROUP BY VEGETABLE_NAME, color ); 

Il metodo sopra funziona se abbiamo una colonna che non fa parte del gruppo per la valutazione dei duplicati. Se tutti i valori nelle colonne hanno copie, non possiamo usare il file ID_VEGETABILE colonna.

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Rilasciamo e creiamo il file verdura tavolo con una nuova struttura.

 DROP TABLE vegetables; CREATE TABLE vegetables ( VEGETABLE_ID NUMBER, VEGETABLE_NAME VARCHAR2(100), Color VARCHAR2(20) ); 

 INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(1,'Potato','Brown'); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(1, 'Potato','Brown'); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color)VALUES(2,'Onion','Red'); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color)VALUES(2,'Onion','Red'); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(2,'Onion','Red'); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(3,'Pumpkin','Green'); INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES('4,Pumpkin','Yellow'); SELECT * FROM vegetables; 

Nella tabella delle verdure, i valori in tutte le colonne ID_VEGETABLE, NOME_VEGETABLE e il colore sono stati copiati.

Possiamo usare il turbolento , un localizzatore che specifica dove Oracle memorizza la riga. Perché il turbolento è unico in modo che possiamo usarlo per rimuovere le righe duplicate.

 DELETE FROM Vegetables WHERE rowed NOT IN ( SELECT MIN(rowid) FROM vegetables GROUP BY VEGETABLE_ID, VEGETABLE_NAME, color ); 

La query verifica l'operazione di cancellazione:

 SELECT * FROM vegetables;