Question

I have two tables like the following :

parameters1

+------+-----+
| cod  | des |
+------+-----+
|  1   | aaa |
|  2   | bbb |
|  3   | ccc |

parameters2

+------+-----+
| cod  | des |
+------+-----+
|  1   | mmm |

I usually JOIN the result set of this two tables with an UNION in this way :

SELECT  cod,  des 
FROM parameters1
UNION 
SELECT  cod,  des 
FROM parameters2

and i get this result set :

+------+-----+
| cod  | des |
+------+-----+
|  1   | aaa |
|  2   | bbb |
|  3   | ccc |
|  1   | mmm |

I want to limit the UNION duplicate checking only to the cod column, so i want to avoid that the 1 cod get duplicated in the result set (when there is a record with different name and the same cod), i want to get the name from the first table (parameters1) :

+------+-----+
| cod  | des |
+------+-----+
|  1   | aaa |
|  2   | bbb |
|  3   | ccc |

UPDATE
If i remove the record 1 from the parameters1 table(DELETE FROM parameters1 WHERE cod = 1) i should get this result set :

+------+-----+
| cod  | des |
+------+-----+
|  1   | mmm |   ---> FROM parameters2
|  2   | bbb |
|  3   | ccc |

Is it possible to limit the duplicate checking of a UNION to only one field or some fields ? How ?

The solution should work on a multidatabase environment (MSSQL, PostgreSQL, MySQL).

Answer 1

SELECT  cod, des 
FROM parameters1 p1
UNION ALL
SELECT  cod, des 
FROM parameters2 p2
WHERE NOT EXISTS (
    SELECT 1
    FROM parameters1 p1sub
    WHERE p1sub.cod = p2.cod
)