Question

My problem is as follows:

I have 3 tables with content:

tableA:|ID_A|field1|field2|field3| => base table


tableB:|ID_B | ID_C |ID_A| => Mapping Table


tableC:|ID_C |FieldToGet| => Additional info field table

* Important assumption: It is possible that there is multiple values of ID_C associated with the same ID_A
such that tableB: has the following structure:`

| ID_B | ID_C | ID_A|

|  24  |  81  |  23 |

|  25  |  11  |  23 |

|  26  | 82   |  23 |

|  27  | 42   |  24 |

|  28  | 113  | 33  |

but if there is to be multiple ID_B for a single ID_A, they will follow each other.

Now, I am writing a php script to fetch the required fied for a certain ID_A, but it fails because my subquery returns multiple rows.:

$q = $this->dao->prepare('SELECT FieldToGet FROM table3  WHERE ID_C = ( SELECT ID_C FROM table2 WHERE ID_A =$SOME_ID_A )');

I am looking for either a solution that only fetches results from columm FieldToGet ( My current attempt) or a solution that will join the whole basetable, with the column FieldToGet ( unsure about how to use the JOIN opearation).

Answer 1

the join will look like this,

SELECT  a.*, c.*   -- select the columns you want to get
FROM    tableA a
        INNER JOIN tableB b
            ON a.ID_A = b.ID_A
        INNER JOIN tableC c
            ON b.ID_C = c.ID_C

To fully gain knowledge about joins, kindly visit the link below:

• Visual Representation of SQL Joins