SQL Exercise: Check if all the employees numbers are indeed unique

SQL employee Database: Exercise-103 with Solution

[An editor is available at the bottom of the page to write and execute the scripts.]

103. From the following table, write a SQL query to check whether the employees ID are unique or not. Return employee id, number of employees.

Sample table: employees

Sample Solution:

SELECT emp_id,
       count(*)
FROM employees
GROUP BY emp_id;

Sample Output:

 emp_id | count
--------+-------
  69324 |     1
  69062 |     1
  63679 |     1
  67858 |     1
  66564 |     1
  64989 |     1
  67832 |     1
  65646 |     1
  68319 |     1
  68454 |     1
  66928 |     1
  65271 |     1
  68736 |     1
  69000 |     1
(14 rows)

Explanation:

The given query in SQL that selects the "emp_id" and counts the number of times each "emp_id" appears in the 'employees' table. It then groups the results by "emp_id".

Relational Algebra Expression:

Relational Algebra Tree:

Practice Online

Sample Database: employee

Have another way to solve this solution? Contribute your code (and comments) through Disqus.

Previous SQL Exercise: List the departments where atleast 2 employees work.
Next SQL Exercise: List the number of employees and average salary.