Counting common elements in MySQL?

To count common elements, use COUNT() and GROUP BY. Let us first create a table −

mysql> create table DemoTable
   (
   Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   Number int
   );
Query OK, 0 rows affected (0.20 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable(Number) values(10);
Query OK, 1 row affected (0.06 sec)

mysql> insert into DemoTable(Number) values(20);
Query OK, 1 row affected (0.04 sec)

mysql> insert into DemoTable(Number) values(20);
Query OK, 1 row affected (0.04 sec)

mysql> insert into DemoTable(Number) values(30);
Query OK, 1 row affected (0.04 sec)

mysql> insert into DemoTable(Number) values(10);
Query OK, 1 row affected (0.05 sec)

mysql> insert into DemoTable(Number) values(10);
Query OK, 1 row affected (0.04 sec)

Display all records from the table using select statement −

mysql> select *from DemoTable;

This will produce the following output −

+----+--------+
| Id | Number |
+----+--------+
| 1  | 10     |
| 2  | 20     |
| 3  | 20     |
| 4  | 30     |
| 5  | 10     |
| 6  | 10     |
+----+--------+
6 rows in set (0.00 sec)

Following is the query to count common elements −

mysql> select Number,count(Number) as CommonElement from DemoTable group by Number;

This will produce the following output −

+--------+---------------+
| Number | CommonElement |
+--------+---------------+
| 10     | 3             |
| 20     | 2             |
| 30     | 1             |
+--------+---------------+
3 rows in set (0.00 sec)