MySQL查询：从其他表中选择所有ID和相应的最大值

Question

What I want: selecting all ID's from table1 and a max value from table2. If table2 doesn't contain an id, 0 is its max value. Query I got so far:

SELECT t1.Id, max(t2.value) FROM table1 t1 JOIN table2 t2 USING(Id) GROUP BY t2.Id;

But the problem with this query is that it only returns an id if it is in both tables. How can I select all id's from table1 and a 0 or a max value from table2?

Answer 1

This is the case to use a LEFT join:

SELECT t1.Id, coalesce(max(t2.value), 0) 
FROM table1 t1 LEFT JOIN table2 t2 
USING(Id) 
GROUP BY t1.Id;

I also changed the GROUP BY clause to use t1.id instead of t2.id .