SQL Learning Post: Does “IN (…) AND IN (…)” Return 20 Rows?

[Rajeev Bagra]

A common beginner confusion in SQL is t

«If one subquery returns 10 rows, and another subquery returns 10 rows, does the final query return 20 rows?»

Short answer:

No — not when using “AND”.

Let’s understand clearly.

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Example Query

SELECT first_name, last_name
FROM players
WHERE id IN (subquery_1)
AND id IN (subquery_2);

Suppose:

• “subquery_1” returns 10 player IDs
• “subquery_2” returns 10 player IDs

Many beginners think:

«10 + 10 = 20 rows»

But SQL does not add the lists together here.

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What “AND” Means

This condition:

WHERE id IN (list1)
AND id IN (list2)

means:

«Keep only rows where the ID exists in both lists.»

That is called the intersection of two sets.

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Visual Example

First List

1, 2, 3, 4, 5, 6, 7, 8, 9, 10

Second List

5, 6, 7, 8, 20, 21, 22, 23, 24, 25

Final Result

5, 6, 7, 8

Only the values present in both lists remain.

So final output = 4 rows, not 20.

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Maximum Possible Rows

If both lists are identical:

1,2,3,4,5,6,7,8,9,10

Then final result = 10 rows maximum.

So with:

IN (…) AND IN (…)

the result range is:

• Minimum: 0 rows
• Maximum: 10 rows

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What If You Used “OR” Instead?

WHERE id IN (list1)
OR id IN (list2)

Now SQL keeps rows from either list.

That is the union of sets.

This could produce up to 20 unique rows.

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Moneyball Example

In CS50 Moneyball:

WHERE id IN (top 10 cheapest per hit)
AND id IN (top 10 cheapest per RBI)

means:

«Find players who are top 10 in both categories.»

So only shared players appear.

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Key SQL Concept

• “AND” = overlap / intersection
• “OR” = combine / union

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Final Rule to Remember

If you see:

IN (…) AND IN (…)

Think:

«“Show me only values that belong to both groups.”»

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