涉及外部联接的mysql中多个表上的联接的SQL count（*）问题

Question

I have three tables, lets say t1, t2, t3 with columns as

 t1.c1, t1.c2, t1.c3 
 t2.c4, t2.c1, t2.c5
 t3.c6, t3.c4 

t1.c1, t2.c4 and t3.c6 are primary, autoincrements fields of their respective tables.

t2.c1 is a foreign key in t2 referring to t1.c1 in t1

t3.c4 is a foreign key in t3 referring to t2.c4 in t2

I want to get the following output for any given value of t2.c5 (say X)

SORRY - Mistake in output columns!

I want output to be

t2.c4, t1.c2, count

and not

t2.c1, t1.c2, count

where count is the number of records in t3 for the given value of t2.c5 and must be zero if no record exists where t2.c5=X and t3.c4=t2.c4

Can this be done in one select query?

Here is sample data:

Table: t1

  +------+-------+------+
  | c1   | c2    | c3   |
  +------+-------+------+
  |    1 | 11111 |  111 |
  |    2 | 22222 |  222 |
  |    3 | 33333 |  333 |
  +------+-------+------+

Table: t2

  +------+------+------+
  | c4   | c1   | c5   |
  +------+------+------+
  |    1 |    1 |   11 |
  |    2 |    1 |   11 |
  |    3 |    2 |   11 |
  |    4 |    2 |   12 |
  |    5 |    3 |   12 |
  |    6 |    2 |   12 |
  +------+------+------+

Table: t3

  +------+------+
  | c6   | c4   |
  +------+------+
  |    1 |    1 |
  |    2 |    1 |
  |    3 |    5 |
  +------+------+

For t2.c5=11 , the output is

    +-------------------+
     t2.c4 | t1.c2| Count
    +-------------------+
      1    | 11111| 2
      2    | 11111| 0
      3    | 22222| 0
    +-------------------+

For t2.c5=12 , the output is

    +-------------------+
     t2.c4 | t1.c2| Count
    +-------------------+
      4    | 22222| 0
      5    | 33333| 1
      6    | 22222| 0

Answer 1

SELECT  t2.c1, t1.c2, count(*)
FROM t1 inner join t2 using (c1)
inner join t3 using (c4)
GROUP BY t2.c5, t2.c1, t1.c2

Whats confusing here is that you want to select t2.c1, t1.c2 for any value of t2.c5.

You will have to decide either grouping by all three columns as shown above, or really grouping by t2.c5, and just showing any value of t2.c2, t1.c2 , which will require using some aggregate function on them. For example:

SELECT  max(t2.c1) c2, max(t1.c2) c2, count(*)
FROM t1 inner join t2 using (c1)
inner join t3 using (c4)
GROUP BY t2.c5

Answer 2

Try this query:

SELECT t2.c4, t1.c2, COUNT(t3.c6) 
FROM t1
INNER JOIN t2 ON t1.c1 = t2.c1
LEFT JOIN t3 ON t3.c4 = t2.c4
WHERE t2.c5 = '11'
GROUP BY t2.c4, t1.c2

Output:

c4  c2  count
-------------
1   11111   2
2   11111   0
3   22222   0

Demo here

With this query:

SELECT t2.c4, t1.c2, COUNT(t3.c6) as count
FROM t1
INNER JOIN t2 ON t1.c1 = t2.c1
LEFT JOIN t3 ON t3.c4 = t2.c4
WHERE t2.c5 = '12'
GROUP BY t2.c4, t1.c2

output is:

c4  c2      count
-----------------
4   22222   0
5   33333   1
6   22222   0

Demo here