(MySQL) How do you get the no of unique values in a column from the resultant table of a Natural Join in a single query?

Question

I have two Tables:

Employee

emp_id, emp_name, emp_dept, emp_sal

1         A           A         1

2         B           B         2

3         C           C         3

4         D           D         4

Trial

emp_id, random

1          X

1          Y

2          Z

3          A

4          B

If I were to perform a Natural join of Employee and Trial,

I would get the result:

emp_id, emp_name, emp_dept, emp_sal random

1         A           A         1      X

1         A           A         1      Y

2         B           B         2      Z

3         C           C         3      A

4         D           D         4      B

If I wanted to perform the aforementioned Natural Join and count the no of depts in the organization in a single query, how would I go about doing that?

select count(emp_dept) from (select distinct(emp_dept) from employee natural join trial) as T2;

works, but I am wondering if there's another way to do the above?

Note:

Natural join is mandatory!

I tried both the following codes and failed miserably. :P

1. select count(emp_dept) from (select (emp_dept) from employee natural join trial) as T2; gave me 5, when the actual answer is 4.

2. select count(select distinct(emp_dept) as emp_date from (select * from employee natural join trial) as T2) from T2;

3. select count(select distinct(emp_dept) as emp_date from (select * from employee natural join trial) as T2) from (select * from employee natural join trial) as T3);

^ What did I do wrong in the 3 lines of code above?

Thanks in advance! :)

Answer 1

Can you not just do a COUNT(DISTINCT .... )? Like this:-

SELECT COUNT(DISTINCT emp_dept) 
FROM employee 
NATURAL JOIN trial