FAQ: Subqueries - Exists and Not Exists

This community-built FAQ covers the “Exists and Not Exists” exercise from the lesson “Subqueries”.

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This exercise can be found in the following Codecademy content:

Analyze Data with SQL

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I’m not quiet get the instruction… “Write a query to find out which grade levels are represented in both band and drama.”.
I’ve looked at the hint. If subquery give us at least one record, we get all the results from main query. As a result - the answer just all grades from the first table. But the exercise is “Write a query to find out which grade levels are represented in BOTH band and drama.”
If I add new student with grade = 1000)) it will be in query result!
The answer must be “9,10,11 and 12”. Isn’t it?
We can do it without EXISTS:

SELECT band_students.grade
FROM band_students
JOIN drama_students
  ON band_students.grade = drama_students.grade 
GROUP BY 1;

With EXISTS we need to add condition:

SELECT grade
FROM band_students
WHERE EXISTS (
  SELECT * 
  FROM drama_students
  WHERE band_students.grade = drama_students.grade 
) 
GROUP BY 1;

Without the condition, we’ll get all rows from outer query again.
If I get it correctly, with inner condition we check every row from band_students table and get TRUE or FALSE in WHERE EXIST statement. If grade in first row of band_students table isn’t equal any grade in drama_students we’ll get FALSE and this grade won’t get into result.

SELECT grade 
FROM band_students
WHERE grade IN(
  SELECT grade
  FROM drama_students
)
GROUP BY 1;

Wouldn’t it be more efficient to use IN clause?

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