Using MySQL group by clause with where clause - mysql

Question

I have two tables, one that store product information and one that stores reviews for the products.
I am now trying to get the number of reviews submitted for the products between two dates but for some reason I get the same results regardless of the dates i put.
This is my query:
SELECT
productName,
COUNT(*) as `count`,
avg(rating) as `rating`
FROM `Reviews`
LEFT JOIN `Products` using(`productID`)
WHERE `date` BETWEEN '2015-07-20' AND '2015-07-30'
GROUP BY
`productName`
ORDER BY `count` DESC, `rating` DESC;
This returns:
+------------+---------------------+
| productName| count|rating |
+------------+------+--------------+
| productA | 23 | 4.3333333 |
| productB | 17 | 4.25 |
| productC | 10 | 3.5 |
+------------+---------------------+
Products table:
+---------+-------------+
|productID | productName|
+---------+-------------+
| 1 | productA |
| 2 | productB |
| 3 | productC |
+---------+-------------+
Reviews table
+---------+-----------+--------+---------------------+
|reviewID | productID | rating | date |
+---------+-----------+--------+---------------------+
| 1 | 1 | 4.5 | 2015-07-27 17:47:01|
| 2 | 1 | 3.5 | 2015-07-27 18:54:22|
| 3 | 3 | 2 | 2015-07-28 13:28:37|
| 4 | 1 | 5 | 2015-07-28 18:33:14|
| 5 | 2 | 1.5 | 2015-07-29 11:58:17|
| 6 | 2 | 3.5 | 2015-07-30 15:04:25|
| 7 | 2 | 2.5 | 2015-07-30 18:11:11|
| 8 | 1 | 3 | 2015-07-30 18:26:23|
| 9 | 1 | 3 | 2015-07-30 21:35:05|
| 10 | 1 | 4.5 | 2015-07-31 14:25:47|
| 11 | 3 | 0.5 | 2015-07-31 14:47:48|
+---------+-----------+--------+---------------------+
when I put two random dates that I do know for sure they not on the date column, I will still get the same results. Even when I want to retrieve records only on a certain day, I get the same results.

Answer 1

You should not use left join, because by doing so you retrieve all the data from one table. What you should use is something like :
select
productName,
count(*) as `count`,
avg(rating) as `rating`
from
products p,
reviews r
where
p.productID = r.productID
and `date` between '2015-07-20' and '2015-07-30'
group by productName
order by count desc, rating desc;

Answer 2

If the result, given your sample data, that you're looking for is:
| productName | count | rating |
|-------------|-------|--------|
| productA | 5 | 4 |
| productB | 3 | 3 |
| productC | 1 | 2 |
This is the count and average of reviews made on any date between 2015-07-20 and 2015-07-30 inclusive.
Then the there are two issues with your query. First, you need to change the join to a inner join instead of a left join, but more importantly you need to change the date condition as you are currently excluding reviews that fall on the last date on the range, but after midnight.
This happens because your between clause compares datetime values with date values so the comparison ends up being date between '2015-07-20 00:00:00' and '2015-07-30 00:00:00' which clearly excludes some dates at the end.
The fix is to either change the date condition so that the end is a day later:
where date >= '2015-07-20' and date < '2015-07-31'
or cast the date column to a date value, which will remove the time part:
where date(date) between '2015-07-20' and '2015-07-30'
Sample SQL Fiddle

Answer 3

You are using a LEFT JOIN between your reviews and your products tables. This will result in all the rows of reviews being shown with some rows having all product columns left empty.
You should use INNER JOIN, as this will filter only the wanted results.
(In the end I can only guess, since I don't even know which column belongs to which table ...)
The full query (very similar to Angelo Giannis's solution):
select
productName,
count(*) as `count`,
avg(rating) as `rating`
from
products INNER JOIN reviews USING(productId)
where date between '2015-07-20' and '2015-07-30'
group by productName
order by count desc, rating desc;
Here a fiddle with my and Angelo's solution (they both work).