﻿ SQL exercises on employee Database: Check whether all the employees numbers are indeed unique - w3resource

# SQL exercises on employee Database: Check whether 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)
```

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.

What is the difficulty level of this exercise?

Test your Programming skills with w3resource's quiz.

﻿

## SQL: Tips of the Day

Delete all Duplicate Rows except for One in MySQL?

```SELECT * FROM names;

+----+--------+
| id | name   |
+----+--------+
| 2  | yahoo  |
| 3  | msn    |
| 6  | yahoo  |
+----+--------+
```

1. If you want to keep the row with the lowest id value:

```DELETE n1 FROM names n1, names n2 WHERE n1.id > n2.id AND n1.name = n2.name
```

2. If you want to keep the row with the highest id value:

```DELETE n1 FROM names n1, names n2 WHERE n1.id < n2.id AND n1.name = n2.name
```

Ref: https://bit.ly/3NroM8o