仅存在一场比赛时的内部加入

Question

Consider the following realistic scenario:

You have a table of towns, but want to lookup the respective zip codes of those towns. In most cases, this is easy to do since there is only one zip code per town, but exceptions do apply when dealing with cities.

You have two tables:

CONTACT
Town          | Zip
--------------+----
Ft Washington | NULL
Ambler        | NULL
Media         | NULL
Ambler        | NULL
New York      | NULL

CITIES_EXTENDED
Town          | Zip
--------------+----
Ft Washington | 19034
Ambler        | 19002
Media         | 19063
New York      | 10101
New York      | 10102
New York      | 10103
New York      | 10104

With an INNER JOIN four rows would be returned for the New York . This is obviously not desirable because that data is inherently inaccurate.

The desired result is as follows:

Town          | Zip
--------------+----
Ft Washington | 19034
Ambler        | 19002
Media         | 19063
Ambler        | 19002
New York      | NULL

What is the best way of achieving this goal?

Answer 1

Does this solve your problem?

select ce.Town, 
case when count(*) > 1 then null else Max(c.Zip) end as Zip
from CITIES_EXTENDED ce
    inner join CONTACT c
        on ce.Town = c.Town
    group by ce.Town

The Max() function will work just fine because you will only have one value for c.Zip.

Answer 2

Try:

Select  a.Town,
        b.zip
FROM    contact a
INNER JOIN cities_extended b ON a.Town = b.Town
WHERE a.Town NOT IN (
    SELECT  Town
    FROM    (
        Select  Town,
                COUNT(*)
        From    cities_extended
        Group by Town
        HAVING COUNT(*) > 1
    ) T
)

UNION ALL 

Select  DISTINCT
        a.Town,
        NULL
FROM    contact a
INNER JOIN  cities_extended b ON a.Town = b.Town
WHERE   a.Town IN (
    SELECT  Town
    FROM    (
        Select  Town,
                COUNT(*)
        From    cities_extended
        Group by Town
        HAVING COUNT(*) > 1
    ) T
)

See my Demo

This may not be an elegant solution but this gets the job done as per your requirement ;-)