MYSQL (MariaDB) - Invalid use of group function - mysql

Question

I have two tables called addresses and house_sales
addresses
+-------------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------------+------------------+------+-----+---------+----------------+
| id | int(11) unsigned | NO | PRI | NULL | auto_increment |
| house_number_name | varchar(150) | NO | | NULL | |
| address_line1 | varchar(150) | NO | MUL | NULL | |
| address_line2 | varchar(150) | YES | | NULL | |
| address_line3 | varchar(150) | YES | MUL | NULL | |
| town_city | varchar(150) | NO | MUL | NULL | |
| district | varchar(150) | YES | MUL | NULL | |
| county | varchar(150) | YES | MUL | NULL | |
| post_code | varchar(8) | NO | MUL | NULL | |
| updated_at | datetime | NO | | NULL | |
| created_at | datetime | NO | | NULL | |
+-------------------+------------------+------+-----+---------+----------------+
house_sales
+---------------+------------------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+------------------------------------------------------------+------+-----+---------+----------------+
| id | int(11) unsigned | NO | PRI | NULL | auto_increment |
| address_id | int(11) unsigned | NO | MUL | NULL | |
| price | int(11) unsigned | NO | MUL | NULL | |
| date | datetime | NO | MUL | NULL | |
| updated_at | datetime | NO | | NULL | |
| created_at | datetime | NO | | NULL | |
+---------------+------------------------------------------------------------+------+-----+---------+----------------+
I'm trying to select all the addresses grouped by address_line1 and then getting the average price for that street. The query works but I want to only select where there is more than one house on the same street. However when I add the AND count(*) > 1 I get the error "Invalid use of group function". Below is the query
SELECT count(*) as total_sales, avg(price) as average_price, `address_line1`, `town_city`
FROM `house_sales` `hs`
LEFT JOIN `addresses` `a` ON `hs`.`address_id` = `a`.`id`
WHERE `town_city` = 'London'
AND count(*) > 1
GROUP BY `address_line1`
ORDER BY `average_price` desc
I'm not sure why I'm getting this error. I've tried a sub query so I can use HAVING but haven't got this to work. Any help or pointers would be appreciated

Answer 1

You need a having clause to filter on the aggregate expression:
SELECT count(*) as total_sales, avg(price) as average_price, `address_line1`, `town_city`
FROM `house_sales` `hs`
LEFT JOIN `addresses` `a` ON `hs`.`address_id` = `a`.`id`
WHERE `town_city` = 'London'
GROUP BY `address_line1`, `town_city`
HAVING count(*) > 1
ORDER BY `average_price` desc
MySQL extends the SQL standard by allowing the use of aliases in the having clause, so you can also do:
having total_sales > 1
Side notes:
as commented by jarlh, it is a good practice to qualify (prefix) all column names with the table they belong to
it is also a good practice to put all non-aggregated columns in the group by clause (I added town_city, which was missing in your original query) - newer versions of MySQL do not allow this by default
quoting all identifiers is usually not necessary (unless they contain special characters)

Answer 2

There are two ways to go here. One would be to add town_city to the GROUP BY list:
SELECT
address_line1,
town_city,
COUNT(*) AS total_sales,
AVG(price) AS average_price
FROM house_sales hs
LEFT JOIN addresses a ON hs.address_id = a.id
WHERE town_city = 'London'
GROUP BY address_line1, town_city
HAVING COUNT(*) > 1
ORDER BY average_price DESC;
The other would be to just keep your current query but remove town_city from the select list, since you are restricting to just London anyway.
SELECT
address_line1,
COUNT(*) AS total_sales,
AVG(price) AS average_price
FROM house_sales hs
LEFT JOIN addresses a ON hs.address_id = a.id
WHERE town_city = 'London'
GROUP BY address_line1
HAVING COUNT(*) > 1
ORDER BY average_price DESC;