﻿ SQL: Count unique patients who came to examination room C

SQL Exercise: Count unique patients who came to examination room C

SQL hospital Database: Exercise-14 with Solution

14. From the following tables, write a SQL query to count the number of unique patients who have been scheduled for examination room 'C'. Return unique patients as "No. of patients got appointment for room C".

Sample table: appointment

Sample Solution:

``````SELECT count(DISTINCT patient) AS "No. of patients got appointment for room C"
FROM appointment
WHERE examinationroom='C';
``````

Sample Output:

```No. of patients got appointment for room C
--------------------------------------------
3
(1 row)
```

Explanation:

The said query in SQL that retrieves the number of unique patients who have had appointments in examination room C.

The WHERE statement ensures that only the appointments in examination room C are counted.

Practice Online

E R Diagram of Hospital Database:

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Previous SQL Exercise: Patients and doctors who gave them preliminary care.
Next SQL Exercise: Patients and the room number where they treated.

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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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