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
SELECT count(*) "Number of unavailable rooms" FROM room WHERE unavailable='true';
Number of unavailable rooms --------------------------- 7 (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".
E R Diagram of 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;
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