The SQL key word DISTINCT has the effect that equivalent data records are output only once. : Distinct « Select Clause « SQL / MySQL

1. SQL / MySQL
2. Select Clause
3. Distinct

The SQL key word DISTINCT has the effect that equivalent data records are output only once.

       
mysql>
mysql>
mysql> CREATE TABLE publishers (
    ->   publID int(11) NOT NULL auto_increment,
    ->   publName varchar(60) collate latin1_german1_ci NOT NULL default '',
    ->   ts timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
    ->   PRIMARY KEY  (publID),
    ->   KEY publName (publName)
    -> ) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_german1_ci;
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql>
mysql> INSERT INTO publishers VALUES (1,'A','2004-12-02 18:36:58'),
    ->                                 (2,'Apress','2004-12-02 18:36:58'),
    ->                                 (3,'New Riders','2004-12-02 18:36:58'),
    ->                                 (4,'O\'Reilly & Associates','2004-12-02 18:36:58'),
    ->                                 (5,'Hanser','2004-12-02 18:36:58');
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql> CREATE TABLE titles (
    ->   titleID int(11),
    ->   title varchar(100),
    ->   subtitle varchar(100),
    ->   edition tinyint(4),
    ->   publID int(11),
    ->   catID int(11),
    ->   langID int(11),
    ->   year int(11),
    ->   isbn varchar(20),
    ->   comment varchar(255),
    ->   ts timestamp,
    ->   authors varchar(255),
    ->   PRIMARY KEY  (titleID)
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql>
mysql>
mysql> INSERT INTO titles VALUES (1,'Linux','Installation',5,1,57,2,2000,NULL,NULL,'2005-02-28 13:34:21','Michael'),
    ->                           (2,'Excel',NULL,NULL,2,3,NULL,2000,NULL,NULL,'2005-02-28 13:34:22','David'),
    ->                           (3,'XML',NULL,NULL,1,2,NULL,1997,NULL,NULL,'2005-02-28 13:34:22','Edwards'),
    ->                           (4,'PHP',NULL,NULL,3,6,NULL,2000,NULL,NULL,'2005-02-28 13:34:22','Tom'),
    ->                           (5,'MySQL','',0,3,34,NULL,2000,'','','2005-02-28 13:34:22','Paul'),
    ->                           (6,'Java',NULL,NULL,4,34,NULL,1999,NULL,NULL,'2005-02-28 13:34:22','Tim');
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql>
mysql> CREATE TABLE rel_title_author (
    ->   titleID int(11),
    ->   authID int(11),
    ->   ts timestamp ,
    ->   PRIMARY KEY  (titleID,authID)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql>
mysql>
mysql> INSERT INTO rel_title_author VALUES (1,1,'2004-12-02 18:37:09'),
    ->                                       (2,1,'2004-12-02 18:37:09'),
    ->                                       (2,2,'2004-12-02 18:37:09'),
    ->                                       (3,3,'2005-02-24 10:46:37'),
    ->                                       (3,4,'2005-02-24 10:46:37'),
    ->                                       (3,5,'2005-02-24 10:46:37'),
    ->                                       (4,6,'2004-12-02 18:37:09');
Query OK, 7 rows affected (0.00 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql> CREATE TABLE authors (
    ->   authID int(11) NOT NULL auto_increment PRIMARY KEY ,
    ->   authName varchar(60),
    ->   ts timestamp
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql>
mysql>
mysql>
mysql> INSERT INTO authors VALUES (1,'Tom','2004-12-02 18:36:51'),
    ->                              (2,'Jack','2004-12-02 18:36:51'),
    ->                              (3,'Jane','2004-12-02 18:36:51'),
    ->                              (4,'Amy','2004-12-02 18:36:51'),
    ->                              (5,'Bob','2004-12-02 18:36:51'),
    ->                              (6,'Race','2004-12-02 18:36:51'),
    ->                              (7,'Green','2004-12-02 18:36:51');
Query OK, 7 rows affected (0.00 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql>
mysql> SELECT DISTINCT publName, authName
    -> FROM publishers, titles, rel_title_author, authors
    -> WHERE titles.titleID = rel_title_author.titleID
    -> AND authors.authID = rel_title_author.authID
    -> AND publishers.publID = titles.publID
    -> ORDER BY publName, authName;
+------------+----------+
| publName   | authName |
+------------+----------+
| A          | Amy      |
| A          | Bob      |
| A          | Jane     |
| A          | Tom      |
| Apress     | Jack     |
| Apress     | Tom      |
| New Riders | Race     |
+------------+----------+
7 rows in set (0.00 sec)

mysql>
mysql>
mysql> drop table publishers;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table titles;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table rel_title_author;
Query OK, 0 rows affected (0.00 sec)

mysql> drop table authors;
Query OK, 0 rows affected (0.00 sec)

   
    
    
    
    
    
    
  

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