到多个MySQL关系中的许多记录（与添加关系的关系）-GROUP BY与JOIN？

Question

i have 3 tables:

# table1 #
## id ##
   1

# table2 #
## id    table1 ##
   1       1
   2       1
   3       1

# table3 #
## id    table2 ##
   1       1
   2       1
   3       2
   4       2
   5       3
   6       3

And query, who join 3 tables in single query

Select COUNT(t2.id)
FROM table1 t1
LEFT OUTER JOIN table2 t2 ON t2.table1 = t1.id
LEFT OUTER JOIN table3 t3 ON t3.table2 = t2.id
WHERE t1.id = 1
GROUP BY t1.id

COUNT(t2.id) should return 3, but i get 6 - I think that is because every record from table2 have 2 records in table3.

Maybe I should change this line?

LEFT OUTER JOIN table3 t3 ON t3.t2 = t2.id

Maybe group this or something? But how do this?

====================================== edit

add to table2 column "price"

# table2 #
## id    table1    price ##
   1       1        10
   2       1        20
   3       1        10

Select SUM(t2.price)
FROM table1 t1
LEFT OUTER JOIN table2 t2 ON t2.table1 = t1.id
LEFT OUTER JOIN table3 t3 ON t3.table2 = t2.id
WHERE t1.id = 1
GROUP BY t1.id

Now, when I want to sum the price I get 80, but I should get 40 - distinct is not good solution, because price is not unique.

Answer 1

Change

Select COUNT(t2.id)

to

Select COUNT(distinct t2.id)

and remove the group by since you select only one group anyway with WHERE t1.id = 1

Answer 2

If this is not exactly that what I need. Suppose - in table2 I have another column "price" and in my query I trying to get this: SUM(t2.price) - then I get double sum - reason as above. Distinct is not good idea, becouse this column don't have unique value. How I can resolve this?

Remove the join to table3:

Select COUNT(t2.id)
FROM table1 t1
  LEFT JOIN table2 t2 ON t2.table1 = t1.id
WHERE t1.id = 1;

Answer 3

When you are joining against t3 you are expanding the rows to 6.

Select sum(t2.price)
FROM table1 t1
LEFT OUTER JOIN table2 t2 ON t2.table1 = t1.id
WHERE t1.id = 1;

Please check SQL Fiddle http://sqlfiddle.com/#!2/2236fb/11