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SQL Exercise: Find the floor with the minimum available rooms

SQL hospital Database: Exercise-27 with Solution

27. From the following tables, write a SQL query to locate the floor with the minimum number of available rooms. Return floor ID as "Floor", Number of available rooms.

Sample table: room


Sample Solution:


-- This SQL query retrieves the count of available rooms grouped by blockfloor, where the count of available rooms for each blockfloor is equal to the lowest total count of available rooms across all blockfloors.

SELECT blockfloor as "Floor", -- Selects the blockfloor column and aliases it as "Floor"
       count(*) AS  "No of available rooms" -- Counts the number of available rooms for each blockfloor and aliases the result column as "No of available rooms"
FROM room -- Specifies the table from which to retrieve data, in this case, the room table
WHERE unavailable='false' -- Filters the rows to include only those where the 'unavailable' column is set to 'false'
GROUP BY blockfloor -- Groups the results by blockfloor
HAVING count(*) = -- Filters the groups to include only those where the count of available rooms is equal to the lowest total count of available rooms across all blockfloors
  (SELECT min(zz) AS highest_total -- Subquery: Calculates the lowest total count of available rooms across all blockfloors
   FROM
     ( SELECT blockfloor , -- Subquery: Selects the blockfloor column
              count(*) AS zz -- Subquery: Counts the number of available rooms for each blockfloor and aliases it as 'zz'
      FROM room -- Subquery: Specifies the table from which to retrieve data, in this case, the room table
      WHERE unavailable='false' -- Subquery: Filters the rows to include only those where the 'unavailable' column is set to 'false'
      GROUP BY blockfloor ) AS t ); -- Subquery: Groups the results by blockfloor and aliases the result set as 't'

Sample Output:

 Floor | No of available rooms
-------+-----------------------
     3 |                     7
     4 |                     7
     2 |                     7
(3 rows)

Explanation:

The said code in SQL that retrieves the number of available rooms on each floor of a building and only returns the floor(s) with the lowest number of available rooms.

The WHERE clause filters the rows in the room table where the unavailable column is equal to 'false', indicating that the room is available.

The GROUP BY clause groups the rows by the blockfloor column, so that the count function is applied to each distinct floor.

The subquery calculates the number of available rooms on each floor, and groups them by floor.

The MIN function in the outer query selects the lowest number of available rooms across all floors.

The HAVING clause compares the count of available rooms on each floor to the highest_total value returned by the subquery. Only the floors with the same count as the highest_total are returned.

Practice Online


E R Diagram of Hospital Database:

E R Diagram: SQL Hospital Database.

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Previous SQL Exercise: Find the floor with the most rooms available.
Next SQL Exercise: Name of the patients, their block, floor, room number.

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