使用Laravel或PHP在mysql表上建立条件关系

Question

I have tables like below.

Table A

id  | val_a
1   | a1
2   | a2
3   | a3

Table B

id  | id_a| val_b | resource_type
1   | 2   | b1    |  1
2   | 2   | b2    |  2
3   | 3   | b3    |  3
4   | 3   | b4    |  3

Table Resource_A

id  |r_val| id_b
1   | ra1 | 1

Table Resource_B

id  |r_val| id_b
1   | rb1 | 2

Table Resource_C

id  |r_val| id_b
1   | rc1 | 3
2   | rc2 | 4

If resource_type is 1 in Table B , then table B make relation with table Resource_A .

If resource_type is 2 in Table B , then table B make relation with table Resource_B .

If resource_type is 3 in Table B , then table B make relation with table Resource_C .

Required Output is:

id_b | id_a| val_b |val_a | resource_type| r_val
 1   | 2   | b1    |  a1  |    1         | ra1 
 2   | 2   | b2    |  a2  |    2         | rb1 
 3   | 3   | b3    |  a3  |    3         | rc1 
 4   | 3   | b4    |  a3  |    3         | rc2

But what is the best way to get it without using loop in laravel?

How Can I achieve this by using Laravel 5.2 OR Laravel 4 OR PHP OR MYSQL Query?

Thanks Ahead.

Answer 1

You only need to join the tables together as I have done below. The tricky part here is bringing in the correct r_val from the three resource tables. We can do this by left joining to each resource table, and then using the following expression to grab the matching value:

COALESCE(t1.r_val, t2.r_val, t3.r_val) AS r_val

This will take the first non NULL value from a resource table, in order of left to right. Assuming that each ID from tableb only appears once, in one of the resource tables, the order of the terms inside COALESCE() should not matter.

SELECT tb.id AS id_b,
       tb.id_a,
       tb.val_b,
       COALESCE(ta.val_a, 'NA') AS val_a,
       tb.resource_type,
       COALESCE(t1.r_val, t2.r_val, t3.r_val) AS r_val
FROM tableb tb
LEFT JOIN tablea ta
    ON tb.id_a = ta.id
LEFT JOIN resource_a t1
    ON tb.id = t1.id_b
LEFT JOIN resource_b t2
    ON tb.id = t2.id_b
LEFT JOIN resource_c t3
    ON tb.id = t3.id_b

Demo here:

SQLFiddle