MySQL 如何进行 3 表连接

Question

I'm trying to perform a 3 table join on MySQL in order to achieve something like the diagram below.

The main problem I'm having is that I only want to work with the records of table A which has 100 records so if there are no relationships for the right tables I would like to see a null.

This all works fine when only table A and B are involved but when I try to do the third join with C I'm getting more than the original 100 records, I'm getting 130 which I believe is because is adding the records that match BC with duplicate data from table A.

What am I missing?

This is the SQL I currently have that returns correctly 100 records

SELECT count(A.id)
FROM tableA A
LEFT JOIN TableB B ON B.id = A.b_id

This is what I'm trying to do that is returning more than the original 100 records for Table A.

SELECT count(A.id)
FROM tableA A
LEFT JOIN TableB B ON B.id = A.b_id
LEFT JOIN TableC C ON C.id = B.c_id

Answer 1

This could be resolved by a JOIN to a subquery rather than a table.

If you had unique Ids to join to, it would simply be like you've tried already (arbitrary example):

SELECT * from table1 t1
LEFT JOIN table2 t2 on t2.id = t1.id
LEFT JOIN table3 t3 on t3.id = t2.id

If, however the id field in table3 wasn't unique, you'd get multiple rows for each duplicate. You could resolve this by:

SELECT * from table1 t1
LEFT JOIN table2 t2 on t2.id = t1.id
LEFT JOIN (SELECT * FROM table3 GROUP BY id) t3 on t3.id = t2.id

So, using your example (assuming only the third join has duplicates), something like:

SELECT count(A.id)
FROM tableA A
LEFT JOIN TableB B ON B.id = A.b_id
LEFT JOIN (SELECT * FROM TableC GROUP BY id) C ON C.id = B.c_id

...should do the trick. This is down to assumption of your table and data structure, so you might want to make the asterisk more explicit.

Answer 2

SELECT count(distinct A.id)
FROM tableA A
LEFT JOIN TableB B ON B.id = A.b_id
LEFT JOIN TableC C ON C.id = B.c_id