Question 114256 in PostgreSQL Question posted in
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Postgresql – can you explain the logic of this query

I have this query that finds the name of the teacher with the 4-th highest salary. I don’t understand this part

SELECT COUNT (DISTINCT T2.salary)
FROM teacher as T2
WHERE T2.salary > T1.salary
) = 3

from

SELECT name
FROM teacher as T1
WHERE (
SELECT COUNT (DISTINCT T2.salary)
FROM teacher as T2
WHERE T2.salary > T1.salary
) = 3;

The way I understand count is that it gives a final result, not that we can interrupt its work by specifying a number.

This is the teacher table: https://imgur.com/a/tZVk1O8
(I couldn’t upload it here due to a server error)

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2

Answers


  1. 0

    I wouldn’t write it that way, but what it does is count, for each teacher, how many salaries are higher than his/her salary.

    If 3 salaries are higher than a given teacher’s salary then that teacher must be ranked 4th.

    The performance of this query will be disastrous with large tables. You should use the rank window function instead.

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  2. 0

    Focusing on the subquery:

    SELECT COUNT(DISTINCT T2.salary)
FROM teacher AS T2
WHERE T2.salary > T1.salary

    This will return the count of distinct teachers having a salary greater than the teacher, in each row of the teacher table. Asserting that this count be equal to 3 means that any matching teacher would have the 4th highest salary (since first 3 positions excluded).

    Note that your logic should behave identically to DENSE_RANK. You could also have used:

    WITH cte AS (
    SELECT *, DENSE_RANK() OVER (ORDER BY salary DESC) rnk
    FROM teacher
)

SELECT name
FROM cte
WHERE rnk = 4;
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