SQL Query Two Tables at once and Loop Result as JSON objects - mysql

Question

I'm not exactly sure what I am looking for here, so apologies if this has already been covered here, I'm not sure what I need to search for!
I have a MySQL database with a table called "Locations" which looks a bit like this
id | name | other parameters
1 | shop1 | blah
2 | shop2 | blah
etc
and a table of customer queries
id | customer | department
1 | john | shop2
2 | Joe | shop2
3 | James | shop1
4 | Sue | shop2
etc
I want to query this and return a JSON object that looks like this
{"location":"shop1","queryCount":"1"},{"location":"shop2","queryCount":"3"}
The location table can be added to with time, and obviously the customer queries will be to, so both need dynamic queries.
I tried this by getting a list of locations by a simple SELECT name from locations query, turning it into an array and then looping through that as follows:
For i = UBound(listofLocations) To 0 Step -1
locations.id = listofLocations(i)
locations.queryCount= RESULT OF: "SELECT COUNT(id) as recordCount from queries WHERE department=listofLocations(i)"
objectArray.Add(locations)
Next
This works, but it is inefficient calling the database through the loop, how do I avoid this?
Thanks

Answer 1

The inefficient is because you are using nested query,
you should use LEFT JOIN, and you just need single query
Here is the SQL:-
select l.name, count(*) as recordCount
from Locations as l
left join customer as c
on l.name = c.department
group by l.id;
And your schema is not very optimized.
Your schema for customer should be
id, customer, location_id <-- using name is redundant,
<-- which should represent in ID (location ID)

Answer 2

First, I must recommend that your "department" field be changed. If you change the spelling of a name in the locations table, your relationships break.
Instead, use the id from the location table, and set up a foreign key reference.
But that's an aside. Using your current structure, this is what I'd do in SQL...
SELECT
location.name,
COUNT(queries.id) AS count_of_queries
FROM
locations
LEFT JOIN
queries
ON queries.department = locations.name
GROUP BY
location.name
Using a LEFT JOIN ensures that you get EVERY location, even if there is no query for it.
Using COUNT(queries.id) instead of COUNT(*) gives 0 if there are no associated records in the queries table.
You can then loop through the result-set of one query, rather than looping multiple queries, and build your JSON string.
It is possible to build the string in SQL, but that's generally considered bad practice. Much better to keep you SQL about data, and you php/whatever about processing and presenting it.

Answer 3

Try this:
SELECT T1.name, (SELECT COUNT(*) FROM queries AS T2 WHERE T2.department = T1.name) AS number FROM locations AS T1;
I agree that your scheme is not proper, you should reference department by Id, not by name

Answer 4

I think you're just looking for "GROUP BY"
SELECT
department_id as location,
COUNT(id) as querycount
FROM
(table of customer queries)
GROUP BY
department_id
As for the database structure... If possible, I would change the tables a bit:
Location:
id | name | other parameters
Customer:
id | name | whatever
table_of_customer_queries:
id | customer_id | location_id
1 | 2 | 2
2 | 4 | 2
3 | 2 | 1
4 | 1 | 2
GROUP BY will only give you results for those departments that have queries. If you want all departments, the LEFT JOIN option mentioned earlier is the way to go.

Answer 5

First off, if your Locations table really stores Department information, rename it as such. Then, change Customer.department to be a fk reference to the Locations.id column (and renamed to department_id), not the name (so much less to get wrong). This may or may not give you a performance boost; however, keep in mind that the data in the database isn't really meant to be human readable - it's meant to be program readable.
In either case, the query can be written as so:
SELECT a.name, (SELECT COUNT(b.id)
FROM Customer as b
WHERE b.department_id = a.id) as count
FROM Location as a

Answer 6

If you don't change your schema, your answer is to do a group by clause that counts the number of queries to each location.
Creating tables like yours:
create table #locations (id integer, name varchar(20), other text)
create table #queries (id integer, customer varchar(20), department varchar(20))
insert into #locations (id, name, other) values (1, 'shop1', 'blah')
insert into #locations (id, name, other) values (2, 'shop2', 'blah')
insert into #queries (id, customer, department) values (1, 'john', 'shop2')
insert into #queries (id, customer, department) values (2, 'Joe', 'shop2')
insert into #queries (id, customer, department) values (3, 'James', 'shop1')
insert into #queries (id, customer, department) values (4, 'Sue', 'shop2')
Querying your data:
select
l.name as location,
count(q.id) as queryCount
from #locations as l
left join #queries as q on q.department = l.name
group by l.name
order by l.name
Results:
location queryCount
-------------------- -----------
shop1 1
shop2 3