# SQL Exercises: Customer with more than one current order

## SQL UNION: Exercise-9 with Solution

9. From the following table, write a SQL query to find those salespersons and customers who have placed more than one order. Return ID, name.

Sample table: customer

Sample table: salesman

Sample table: orders

Sample Solution:

``````SELECT customer_id as “ID”,  cust_name as “NAME”
FROM customer a
WHERE 1<
(SELECT COUNT (*)
FROM orders b
WHERE a.customer_id = b.customer_id)
UNION
(SELECT salesman_idas “ID”,   nameas “NAME”
FROM salesman a
WHERE 1 <
(SELECT COUNT (*)
FROM orders b
WHERE  a.salesman_id = b.salesman_id))
ORDER BY 2
``````

Sample Output:

```ID	        NAME
3009		Geoff Cameron
3005		Graham Zusi
5001		James Hoog
5003		Lauson Hen
5002		Nail Knite
3002		Nick Rimando
```

Code Explanation:

The said query in SQL which selects customer_id and cust_name columns from the customer table and renames them as "ID" and "NAME" respectively.
The subquery checks whether at least one order placed by each customer or not. If the count of orders for a customer is greater than 1, then that customer is included in the results.
The UNION operator is used to combine the results of this query with another query that selects salesman_id and name columns from the salesman table, renames them as "ID" and "NAME" respectively, and checks if there is at least one order placed by each salesman. The final result is then ordered by the "NAME" column.

## Query Visualization:

Duration:

Rows:

Cost:

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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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