In the world of data science, finding the right candidates with the exact skill set is crucial. Imagine you’re tasked with identifying candidates from a database who possess all the necessary skills for a data science job. Specifically, you’re looking for candidates proficient in Python, Tableau, and PostgreSQL. Let’s break down this problem and craft an SQL query to solve it.

Problem Statement

Given a table named candidates, which lists candidate IDs and their skills, you need to find candidates who have all the required skills: Python, Tableau, and PostgreSQL. The output should be a list of candidate IDs sorted in ascending order.

Table: candidates

Example Input:

Example Output:

In this output:

• Candidate 123 is listed because they have all the required skills: Python, Tableau, and PostgreSQL.
• Candidate 345 is not included because they are missing the skill PostgreSQL.

Step-by-Step Solution

1. Filter for Required Skills: Start by filtering the table to include only those rows where the skill is one of the required ones (Python, Tableau, or PostgreSQL).
2. Group and Count Skills per Candidate: Group the filtered results by candidate ID and count the number of unique skills each candidate has.
3. Select Candidates with All Required Skills: Only select those candidates who have exactly 3 unique skills, indicating they possess all the required skills.
4. Sort the Output: Ensure the result is sorted by candidate_id in ascending order.

SQL Query

Here’s how you can write the SQL query to achieve this:

SELECT candidate_id
FROM candidates
WHERE skill IN ('Python', 'Tableau', 'PostgreSQL')
GROUP BY candidate_id
HAVING COUNT(DISTINCT skill) = 3
ORDER BY candidate_id;

Explanation of the Query

1. Filtering (WHERE clause):

• The WHERE skill IN ('Python', 'Tableau', 'PostgreSQL') condition filters the candidates table to include only the rows where the skill is either Python, Tableau, or PostgreSQL.

1. Grouping and Counting (GROUP BY and HAVING clauses):

• The GROUP BY candidate_id groups the rows by candidate_id.
• The HAVING COUNT(DISTINCT skill) = 3 ensures that only candidates who have exactly 3 distinct skills are selected. This is crucial as it checks that the candidate possesses all three required skills.

1. Sorting (ORDER BY clause):

• ORDER BY candidate_id sorts the final output by candidate ID in ascending order.

Hints and Tips for Solving Similar Problems

• Understand the Problem: Before jumping into writing SQL, break down the problem into smaller steps. In this case, the goal was to find candidates with all the required skills.
• Use Aggregations: In scenarios where you need to group data and apply conditions, use GROUP BY and HAVING.
• Filtering with WHERE Clause: When you need to focus on specific values, the WHERE clause is your friend.
• Practice Different Approaches: Try solving similar problems to strengthen your SQL skills. If this problem seems tricky, look for SQL roadmaps and practice resources to build a solid foundation.

Conclusion

This problem is a great example of how SQL can be used to filter and aggregate data to find precise results. It’s a common pattern in job matching, candidate filtering, and other data analysis tasks. By mastering these SQL techniques, you’ll be well-prepared to tackle similar real-world challenges in data science and analytics.