SQL Problem 1 — Matching Skills — Easy Solution

Running notes

Tables we have -

candidate | skill

Question

1. find candidates proficient in Python, Tableau, and PostgreSQL (ALL 3 SKILLS)
2. Sort the output by candidate ID in ascending order

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Solution

SELECT * 
FROM candidates;

This has given me an overall view of the database, so i know how the dataset looks like

SELECT * 
FROM candidates
WHERE skill IN ('Python','Tableau','PostgreSQL');

This is now showing me all the candidates that have any one of these skills

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WHERE clause outputs the rows where a certain condition is met

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IN clause

• used to filter categorical data
• instead of a more tedious approach of multiple ORs we can directly use an IN clause
• so here it is checking if anything from the ‘skill’ column is matching to either - Python, Tableau or PostgreSQL and if any of the value is matched that row is shown in the output

Now the question further says that I need to find those candidate_id’s who have all the 3 skills

SELECT candidate_id, COUNT(skill) as number_of_skills_out_of_3_required_skills
FROM candidates
WHERE skill IN ('Python','Tableau','PostgreSQL')
GROUP BY candidate_id;

So in order to group by the candidates and count the number of skills they have from the 3 required skills

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GROUP BY clause tells the database to separate the data in different groups so we can perform aggregation on these groups separately (like how I need to perform an aggregation on the number of skills to see if they have all the 3 required skills)

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COUNT counts how many rows we have in a particular column

so this is how my output looks like so now I know candidate 123 has all the 3 required skills but candidate 345 has only 2 required skills

so my output should only show candidate_id 123

SELECT candidate_id, COUNT(skill) as number_of_skills_out_of_3_required_skills
FROM candidates
WHERE skill IN ('Python','Tableau','PostgreSQL')
GROUP BY candidate_id
WHERE COUNT(skill) = 3;

So a general thought would lead us to using a WHERE clause (WHERE clause outputs only a certain rows where the condition is satisfied)

This code will give an error because aggregate functions are not allowed in WHERE clause

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WHERE clause is used to filter data before aggregation

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To filter data based on an aggregate function result, we must use the HAVING clause.

So, drumrolls for the final query

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

Written By,

Harshee Pitroda