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Selecionando as principais entradas que ocorrem no MySQL de uma tabela com valores duplicados?


Vamos primeiro criar uma tabela:

mysql> create table DemoTable610 (SubjectName varchar(100));
Query OK, 0 rows affected (0.63 sec)

Insira alguns registros na tabela usando o comando insert:

mysql> insert into DemoTable610 values('MySQL');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable610 values('Java');
Query OK, 1 row affected (0.23 sec)
mysql> insert into DemoTable610 values('MySQL');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable610 values('MongoDB');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable610 values('MySQL');
Query OK, 1 row affected (0.23 sec)
mysql> insert into DemoTable610 values('MySQL');
Query OK, 1 row affected (0.08 sec)
mysql> insert into DemoTable610 values('Java');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable610 values('MongoDB');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable610 values('C');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable610 values('C++');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable610 values('MongoDB');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable610 values('MySQL');
Query OK, 1 row affected (0.21 sec)
mysql> insert into DemoTable610 values('Java');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable610 values('Python');
Query OK, 1 row affected (0.08 sec)
mysql> insert into DemoTable610 values('MySQL');
Query OK, 1 row affected (0.15 sec)

Exiba todos os registros da tabela usando a instrução select:

mysql> select *from DemoTable610;

Isso produzirá a seguinte saída:

+-------------+
| SubjectName |
+-------------+
| MySQL       |
| Java        |
| MySQL       |
| MongoDB     |
| MySQL       |
| MySQL       |
| Java        |
| MongoDB     |
| C           |
| C++         |
| MongoDB     |
| MySQL       |
| Java        |
| Python      |
| MySQL       |
+-------------+
15 rows in set (0.00 sec)

Aqui está a consulta para selecionar apenas as entradas mais frequentes:

mysql> select SubjectName from ( select SubjectName from DemoTable610 group by SubjectName order by count(SubjectName) DESC LIMIT 2 ) AS tbl order by SubjectName;

Isso produzirá a seguinte saída:

+-------------+
| SubjectName |
+-------------+
| Java        |
| MySQL       |
+-------------+
2 rows in set (0.04 sec)

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