wrapping inside aggregate function in SQL query - mysql

Question

I have 2 tables called Orders and Salesperson shown below:
And I want to retrieve the names of all salespeople that have more than 1 order from the tables above.
Then firing following query shows an error:
SELECT Name
FROM Orders, Salesperson
WHERE Orders.salesperson_id = Salesperson.ID
GROUP BY salesperson_id
HAVING COUNT( salesperson_id ) >1
The error is:
Column 'Name' is invalid in the select list because it is
not contained in either an aggregate function or
the GROUP BY clause.
From the error and searching it on google, I could understand that the error is because of Name column must be either a part of the group by statement or aggregate function.
Also I tried to understand why does the selected column have to be in the group by clause or art of an aggregate function? But didn't understand clearly.
So, how to fix this error?

Answer 1

SELECT max(Name) as Name
FROM Orders, Salesperson
WHERE Orders.salesperson_id = Salesperson.ID
GROUP BY salesperson_id
HAVING COUNT( salesperson_id ) >1
The basic idea is that columns that are not in the group by clause need to be in an aggregate function now here due to the fact that the name is probably the same for every salesperson_id min or max make no real difference (the result is the same)
example
Looking at your data you have 3 entry's for Dan(7) now when a join is created the with row Dan (Name) gets multiplied by 3 (For every number 1 Dan) and then the server does not now witch "Dan" to pick cos to the server that are 3 lines even doh they are semantically the same
also try this so that you see what I am talking about:
SELECT Orders.Number, Salesperson.Name
FROM Orders, Salesperson
WHERE Orders.salesperson_id = Salesperson.ID
As far as the query goes INNER JOIN is a better solution since its kinda the standard for this simple query it should not matter but in some cases can happen that INNER JOIN produces better results but as far as I know this is more of a legacy thing since this days the server should pretty much produce the same execution plan.
For code clarity I would stick with INNER JOIN

Answer 2

Assuming the name is unique to the salesperson.id then simply add it to your group by clause
GROUP BY salesperson_id, salesperson.Name
Otherwise use any Agg function
Select Min(Name)
The reason for this is that SQL doesn't know whether there are multiple name per salesperson.id

Answer 3

For readability and correctness, I usually split aggregate queries into two parts:
The aggregate query
Any additional queries to support fields not contained in aggregate functions
So:
1.Aggregate query - salespeople with more than 1 order
SELECT salesperson_id
FROM ORDERS
GROUP BY salespersonId
HAVING COUNT(Number) > 1
2.Use aggregate as subquery (basically a select joining onto another select) to join on any additional fields:
SELECT *
FROM Salesperson SP
INNER JOIN
(
SELECT salesperson_id
FROM ORDERS
GROUP BY salespersonId
HAVING COUNT(Number) > 1
) AGG_QUERY
ON AGG_QUERY.salesperson_id = SP.ID
There are other approaches, such as selecting the additional fields via aggregation functions (as shown by the other answers). These get the code written quickly so if you are writing the query under time pressure you may prefer that approach. If the query needs to be maintained (and hence readable) I would favour subqueries.