How to correctly select data from two tables?

Question

MySQL

There are two tables of the following form:
A:

• id
  
• x
  
• y
  

B:

• a_id
  
• z
  

It is necessary to select all data from table A and the sum of all Bz for A.id. If there are no records with the required a_id, select the sum as 0.
Query like:

select A.*, SUM(B.z) from A,B where A.id = B.a_id GROUP BY A.id;

Doesn't return rows from table A that don't have a match in table B.
How can this be dealt with?

Answer 1

Use LEFT JOIN

Answer 2

Use a nested query
here you will find the necessary information, well, or join)

Answer 3

select
 t1.id,
 t1.x,
 t1.y,
 ifnull(sum(t2.z),0) 
 from A t1
 left join B t2 on t2.a_id = t1.id
 group by t1.id,t1.x,t1.y