除非为空，否则如何基于一列联接两个表，在这种情况下，应基于两个不同的列联接

Question

So this is confusing, but lets start with the following tables:

 Table1           Table2
 ID1  |  ID2  ||  ID1  |  ID2
 -----------  ||  -----------
 jve  |  234  ||  jve  |  null
 null |  234  ||  dav  |  234
 rev  |  584  ||  rev  |  null
 null |  584  ||  ewj  |  584
 avv  |  442  ||  avv  |  null
 null |  442  ||  pol  |  442

*The tables are organized for ease of understanding but aren't in reality

What I'm trying to do is join Table1 and Table2 on T1.ID1 = T2.ID1. However, if ID1 happens to be null, then I want to join them on T1.ID2 = T2.ID2 and not the first way.

What I've gotten up to with googling is

SELECT *
  FROM table1 t1
       INNER JOIN table2 t2
          ON (    t1.ID1 = COALESCE (t2.ID1, t1.ID1)
              AND t1.ID2 = COALESCE (t2.ID2, t1.ID2))
 WHERE ID2 = '234';

The results I'm trying to get to pop up are just to horizontally merge the above tables as such:

Results
-------
jve  | 234 | jve | null
null | 234 | dav | 234
etc..

The reason for this is that there are many more columns beyond ID2 that I'm trying to align between table 1 and table 2.

The main struggle here is I'm not sure how to put the logic,

if( T1.ID1 == null )
{
   ON T1.ID2 = T2.ID2
}
else
{
   ON T1.ID1 = T2.ID1
}

into SQL

If more information is needed, I will gladly add to my post.

I forgot to include that if it joins the first way, it shouldn't join the second way afterwards. What I mean by this, is that first row, jve | 234 should join to jve | null and then the next row, null | 234 should be grabbed instead of joining jve | 234 to dav | 234... if that made any sense..

I need these two tables to join using one method or the other, but never both.

Answer 1

Your logic sounds like an OR condition in the ON clause:

SELECT *
FROM table1 t1 INNER JOIN
     table2 t2
     ON t1.ID1 = t2.ID1 OR
        (t1.ID1 IS NULL AND t1.ID2 = t2.ID2)
WHERE t1.ID2 = '234';

Answer 2

You could do a double-left-join to the second table and apply a COALESCE so it something like

SELECT 
      t1.ID1,
      t1.ID2,
      COALESCE( t2a.ID1, t2b.ID1 ) as T2ID1,
      COALESCE( t2a.ID2, t2b.ID2 ) as T2ID2   
   FROM 
      table1 t1
         LEFT JOIN table2 t2a
            ON t1.ID1 = t2a.ID1
           AND NOT t1.ID1 IS NULL
         LEFT JOIN table2 t2b
            ON t1.ID2 = t2b.ID2
           AND t1.ID1 IS NULL
   WHERE 
      t1.ID2 = '234'

So, if the first table T2A has a record, it pulls the values from that row respectively. If it is null, then it goes to T2B alias for the fields.