Question

In the returned results on a query to a ‘Links’ table I want to also add an extra column that shows a ‘DisplayName’ value that is selected from a JOIN conditional on the value of the ‘toTable’ per row. In addition sometimes that ‘DisplayName’ value is the result of a concatenation of columns.

My question is how do I do this?

I can write seperate queries and combine with UNION ALL though I wonder if there is a conditional way to do it

Each entry in the ‘Links’ table describes links between a pair of entries in two tables (of a possible 3 or more tables).

Links Table:

| LinkID | fromTable | fromID | toTable | toID |
|      1 |        A  |      1 |      B  |    1 |
|      2 |        A  |      1 |      C  |    2 |
|      3 |        B  |      1 |      C  |    1 |
|      4 |        C  |      1 |      A  |    1 |

Query results I want:
WHERE Links.fromTable = ‘A’ OR Links.fromTable = ‘C’ for example

| LinkID | fromTable | fromID | toTable | toID | DisplayName      |
|      1 |        A  |      1 |      B  |    1 |  Some Title      |
|      2 |        A  |      1 |      C  |    2 |  Loud-Quack      |
|      4 |        C  |      1 |      A  |    1 |  Linus           |

3 Tables with different Columns:

Table A:    
| ID  | Name  |
| 1   | Linus |

–

Table B:    
| ID  | Title      |
|  1  | some title |

–

Table C:    
| ID  | CategoryA | CategoryB |
| 1   |   Bark    |    Softly |
| 2   |   Quack   |    Loud   |

–
This is how I am doing this will UNION ALL which is not very flexible:
EDIT: actually i think the below is wrong, i am looking at this now:

SELECT Links.*, A.Name AS DisplayName
FROM Links 
JOIN A ON Links.toID = A.ID 
WHERE Links.fromType IN ('A') AND Links.toType IN ('B')

UNION ALL

SELECT Links.*, CONCAT(C.CategoryB,'-',C.CategoryA) AS DisplayName
FROM Links 
JOIN C ON Links.toID = C.ID 
WHERE Links.fromType IN ('A') AND Links.toType IN ('C')

Answer 1

I thin you need inline CASE statement

SELECT  a.*,
        CASE toTable
            WHEN 'A' THEN b.Name
            WHEN 'B' THEN c.Title
            WHEN 'C' THEN CONCAT(d.CategoryB , '-',d.CategoryA )
        END AS DisplayName
FROM    `links` a
            LEFT JOIN tableA b
                ON a.toID = b.ID
            LEFT JOIN tableB c
                ON a.toID = c.ID
            LEFT JOIN tableC d
                ON a.toID = d.ID
WHERE   fromtable IN ('A', 'C')

SQLFiddle Demo