SQL - select * given count from another table - sql

Question

I'm trying to select * from two tables (a and b) using a join (column a.id and b.id), given that the count of a column (b.owner) in b is lower than 3, i.e. the occurence of a person's name can be max 2.
I've tried:
SELECT a.*, COUNT(b.owner) AS b_count
FROM a LEFT JOIN b on a.id = b.id
GROUP BY b.owner HAVING COUNT(b_count) <3
As im pretty new to SQL, im pretty stuck here. How can i resolve this issue? The result should be all columns for owners who do not appear more than twice in the data.

Answer 1

The query you are trying to run is not working due to the columns missing in the GROUP BY clause.
As you are outputting all columns from table a (with SELECT a.*), you need to include all those columns in the GROUP BY statement, so that the database understand the group of fields to group by and perform the aggregation required (in your case COUNT(b.owner)).
Example
Considering that your table a has 3 columns below:
CREATE TABLE persons (
id INTEGER,
name VARCHAR(50),
birthday DATE,
PRIMARY KEY (id)
);
.. and your table b the following and referencing the first table as below:
CREATE TABLE sales (
id INTEGER,
person_id INTEGER,
sale_value DECIMAL,
PRIMARY KEY (id),
FOREIGN KEY (person_id) REFERENCES persons(id)
);
.. you should query it aggregating the COUNT() by those 3 columns:
SELECT a.id, a.name, a.birthday, COUNT(b.person_id) AS b_count
FROM persons a
LEFT JOIN sales b ON a.id = b.person_id
GROUP BY a.id, a.name, a.birthday
HAVING COUNT(b.person_id) < 3
Alternative
In case the total of records on the 2nd table is not important to you, you could use a different "strategy" here to avoid performing the JOIN between the tables (useful when joining two huge tables) and rewriting all the columns from a on the SELECT+GROUP BY.
By identifying the records that has less than the 3 occurrences firstly:
SELECT b.person_id
FROM sales b
GROUP BY b.person_id
HAVING COUNT(b.id) < 3;
.. and using it in the WHERE clause to retrieve all the columns from the 1st table only for the ids that resulted from the previous query:
SELECT a.*
FROM persons a
WHERE a.id IN (....other query here....);
.. the execution happens in a more chronological and, perhaps, easier way to visualize while getting more familiar with SQL:
SELECT a.*
FROM persons a
WHERE a.id IN (SELECT b.person_id
FROM sales b
GROUP BY b.person_id
HAVING COUNT(b.id) < 3);
DB Fiddle here

Answer 2

In Standard SQL, you can use:
SELECT a.*, COUNT(b.owner) AS b_count
FROM a LEFT JOIN
b
ON a.id = b.id
GROUP BY a.id
HAVING COUNT(b.owner) < 3;
This may not work in all databases (and it assumes that a.id is unique/primary key). An alternative would be to use a correlated subquery:
SELECT a.*
FROM (SELECT a.*,
(SELECT COUNT(*)
FROM b
WHERE a.id = b.id
) as b_count
FROM a
) a
WHERE b_count < 3;