如何运行以下查询？

Question

I have to table A and B. The relationship is one to many. In table B have foreign key from A.

TABLE A:
   ... fields

TABLE B:
   f = foreignkey(A)

How to get all A without B ?

The query below is not working.

select TABLE_A.id, COUNT(TABLE_B.f)  
from TABLE_A JOIN TABLE_B 
   ON (TABLE_A.id = TABLE_B.f) 
GROUP BY TABLE_A.id HAVING COUNT(TABLE_B.f) = 0; 

Answer 1

Change JOIN to LEFT JOIN :

TABLE_A.id, COUNT(TABLE_B.f) 
from TABLE_A 
LEFT JOIN TABLE_B ON (TABLE_A.id = TABLE_B.f) 
GROUP BY TABLE_A.id 
HAVING COUNT(TABLE_B.f) = 0;

Answer 2

Use not exists ?

select a.*
from table_a a
where not exists (select 1
                  from table_b b
                  where a.id = b.f
                 );

Or not in . Or left join with where .

Answer 3

You can do a LEFT JOIN and take only the rows that have NULL in the column TABLE_B.f ,
meaning that there is no TABLE_B.f to match TABLE_A.id :

SELECT TABLE_A.id  
FROM TABLE_A LEFT JOIN TABLE_B 
ON TABLE_A.id = TABLE_B.f 
WHERE TABLE_B.f IS NULL;

assuming that TABLE_A.id is unique in TABLE_A ,
otherwise use

SELECT DISTINCT TABLE_A.id  

Answer 4

You can use the left join and check the nullity on right table.

SELECT TABLE_A.id,count(*)  
FROM TABLE_A LEFT JOIN TABLE_B 
ON TABLE_A.id = TABLE_B.f 
WHERE TABLE_B.f IS NULL
group by TABLE_A.id;