如何显示一个表中的所有记录，即使它们与JOIN不匹配WHERE子句

Question

I have a problem with a consult im trying to achieve :

SELECT [columns] FROM table1 LEFT/RIGHT JOIN table2 ON table1.id_user = table2.id  [WHERE clause] GROUP BY column

An example of what I want this consult to do is shown below:

Having this records in table1 :

id | id_user | user | value

1 | 1 | name1 | 10

2 | 1 | name1 | 10

3 | 1 | name1 | 11

4 | 1 | name1 | 10

5 | 2 | name2 | 11

6 | 2 | name2 | 11

And this ones in table 2:

id | user

1 | name1

2 | name2

I would like to get how many records exist of every user given a value, for exmample if we look for value 10 I would like to get :

name1 | 3

name2 | 0

I've tried with LEFT and RIGHT JOIN but didn't work, I just got this as are the only records that match the WHERE clause i guess:

name1 | 3

And not result for name2.

I've been unsuccessfully looking for a solution before taking my problem here, hope someone can help me with this issue :)

Sorry for the bad english. I really appreciate any help you can provide

Answer 1

not the most efficient query but this should work

select user, (select count(*) from table1 b where b.id_user=a.id and b.value=10)
from table2 a

often, because this query is so inefficient, i will join the results on the client instead of trying to do it in sql. the process would go something like this.

1. select * from table2
2. select id_user, count(*) from table1 where value=10
3. iterate through query #1 and store the results in an object that might look like {'user': name, 'count': 0}
4. iterate through query #2 and update the objects from step #3.

it always depends on the size of the result sets and maybe hardware involved, but often this has been faster albeit quite a bit more complex to write in the code.

Answer 2

Try this.

SELECT t2.user,SUM(CASE WHEN t2.id=t1.id_user THEN 1 ELSE 0 END) AS TotalRecords 
FROM table2 t2
LEFT JOIN table1 t1 ON t2.id =t1.id_user
GROUP BY t2.id 
HAVING t1.value=10