How to do a full outer join without having full outer join available

Question

Last week I was surprised to find out that sybase 12 doesn't support full outer joins. But it occurred to me that a full outer join should be the same as a left outer join unioned with a right outer join of the same sql. Can anybody think of a reason this would not hold true?

Answer 1

UNION-ing two OUTER JOIN statements should result in duplicate rows representing the data you'd get from an INNER JOIN. You'd have to probably do a SELECT DISTINCT on the data set produced by the UNION. Generally if you have to use a SELECT DISTINCT that means it's not a well-designed query (or so I've heard).

Answer 2

If you union them with UNION ALL, you'll get duplicates. If you just use UNION without the ALL, it will filter duplicates and therefore be equivalent to a full join, but the query will also be a lot more expensive because it has to perform a distinct sort.

Answer 3

UNION ALL the left join with the right join, but limit the right join to only rows that do not exist in the base table (return null on the join when they would not be null in the table if they existed).

For this code you will need to create two tables t1 and t2. t1 should have one column named c1 with five rows containing the values 1-5. t2 should also have a c1 column with five rows containing the values 2-6.

Full Outer Join:

select * from t1 full outer join t2 on t1.c1=t2.c1 order by 1, 2;

Full Outer Join Equivalent:

select t1.c1, t2.c1 from t1 left join  t2 on t1.c1=t2.c1
union all
select t1.c1, t2.c1 from t1 right join t2 on t1.c1=t2.c1 
where t1.c1 is null
order by 1, 2;

Note the where clause on the right joined select that limits the results to only those that would not be duplicates.

Answer 4

1. Well first, I don't know why you are using 12.x. It was EndOfLifed on 31 Dec 2009, after having been notified on 3 Apr 2007. 15.0.2 (first solid version) came out in Jan 2009. 15.5 is much better and was available 02 Dec 2009, so you are two major releases, and over at least 13 months, out of date.

2. ASE 12.5.4 has the new Join syntax. (you have not specified, you may be on 12.5.0.3, the release prior to that).

3. DB2 and Sybase did not implement FULL OUTER JOIN, for precisely the reason you have identified: it is covered by LEFT ... UNION ... RIGHT without ALL. It is not a case of "not supporting" a FOJ; it is a case of the keyword is missing.

4. And then you have the issue that Sybase and DB2 types would generally never use outer joins let alone FOJs, because their databases tend to be more normalised, etc.

5. Last, there is perfectly ordinary SQL you can use in any version of Sybase that will supply the function of FOJ, and will be distinctly faster on 12.x; only marginally faster on 15.x. It is kind of like the RANK() function: quite unnecessary if you can write a Subquery.

6. The second reason it does not need FULL OUTER, as some of the low end engines do, is because the new optimiser is extremely fast, and the query is fully normalised. Ie. it performs the LEFT and the RIGHT in a single pass.

7. Depending on you SARGs and DataType mismatches, etc it may still have to sort-merge, but that too is streamed at all three levels: disk I/O subsystem; engine(s); and network handler. If your tables are partitioned, then it is additionally parallelised at that level.

8. If your server is not configured and your result set is very large, you may need to increase proc cache size and number of sort buffers. That's all.