SQL Exercise: Patients with at least one physician appointment
SQL hospital Database: Exercise-4 with Solution
4. From the following table, write a SQL query to count the number of patients who scheduled an appointment with at least one physician. Return count as "Number of patients taken at least one appointment".
Sample table: appointment
Sample Solution:
SELECT count(DISTINCT patient) AS "No. of patients taken at least one appointment"
FROM appointment;
Sample Output:
No. of patients taken at least one appointment ------------------------------------------------ 4 (1 row)
Explanation:
The given query in SQL that selects the number of distinct patients who have taken at least one appointment from a table 'appointment'.
The uses of COUNT and DISTINCT functions in combination with the patient column counts the number of unique patients in the table.
The query will return only a single row with a single column.
The AS keyword is used to provide a column alias, which renames the column to "No. of patients taken at least one appointment" for easier readability .
Pictorial presentation:

Practice Online
E R Diagram of Hospital Database:

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Previous SQL Exercise: Physicians who is the head of each department.
Next SQL Exercise: Find the floor and block with a given room number.
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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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