SQL Exercise: Count the number of unavailable rooms

SQL hospital Database: Exercise-7 with Solution

7. From the following table, write a SQL query to count the number of unavailable rooms. Return count as "Number of unavailable rooms".

Sample table: room

Sample Solution:

SELECT count(*) "Number of unavailable rooms"
FROM room
WHERE unavailable='true';

Sample Output:

Number of unavailable rooms
(1 row)


The said query in SQL that counts the number of unavailable rooms in the room table.

The query selects all columns (*) from the room table where the unavailable column is set to 'true'. The resulting output will be a single row with a single column, showing the number of unavailable rooms in the room table. The column will be labeled "Number of unavailable rooms".

Pictorial presentation:

Count the number of unavailable rooms

Practice Online

E R Diagram of Hospital Database:

E R Diagram: SQL Hospital Database.

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SQL: Tips of the Day

Grouped LIMIT in PostgreSQL: Show the first N rows for each group?

db=# SELECT * FROM xxx;
 id | section_id | name
  1 |          1 | A
  2 |          1 | B
  3 |          1 | C
  4 |          1 | D
  5 |          2 | E
  6 |          2 | F
  7 |          3 | G
  8 |          2 | H
(8 rows)

I need the first 2 rows (ordered by name) for each section_id, i.e. a result similar to:

id | section_id | name
  1 |          1 | A
  2 |          1 | B
  5 |          2 | E
  6 |          2 | F
  7 |          3 | G
(5 rows)

PostgreSQL v9.3 you can do a lateral join

select distinct t_outer.section_id, t_top.id, t_top.name from t t_outer
join lateral (
    select * from t t_inner
    where t_inner.section_id = t_outer.section_id
    order by t_inner.name
    limit 2
) t_top on true
order by t_outer.section_id;

Database: PostgreSQL

Ref: https://bit.ly/3AfYwZI


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