﻿ SQL: Patients with at least one physician appointment

# 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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## 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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