I Need some sort of Conditional Join.

Question

Okay, I know there are a few posts that discuss this, but my problem cannot be solved by a conditional where statement on a join (the common solution).

I have three join statements, and depending on the query parameters, I may need to run any combination of the three. My Join statement is quite expensive, so I want to only do the join when the query needs it, and I'm not prepared to write a 7 combination IF..ELSE.. statement to fulfill those combinations.

Here is what I've used for solutions thus far, but all of these have been less than ideal:

LEFT JOIN joinedTable jt
ON jt.someCol = someCol
WHERE jt.someCol = conditions
OR @neededJoin is null

(This is just too expensive, because I'm performing the join even when I don't need it, just not evaluating the join)

OUTER APPLY
(SELECT TOP(1) * FROM joinedTable jt 
WHERE jt.someCol = someCol
AND @neededjoin is null)

(this is even more expensive than always left joining)

SELECT @sql = @sql + ' INNER JOIN joinedTable jt ' +
             ' ON jt.someCol = someCol ' +
             ' WHERE (conditions...) '

(this one is IDEAL, and how it is written now, but I'm trying to convert it away from dynamic SQL).

Any thoughts or help would be great!

EDIT: If I take the dynamic SQL approach, I'm trying to figure out what would be most efficient with regards to structuring my query. Given that I have three optional conditions, and I need the results from all of them my current query does something like this:

IF condition one
SELECT from db
INNER JOIN condition one

UNION

IF condition two
SELECT from db
INNER JOIN condition two

UNION

IF condition three
SELECT from db
INNER JOIN condition three

My non-dynamic query does this task by performing left joins:

SELECT from db
LEFT JOIN condition one
LEFT JOIN condition two
LEFT JOIN condition three
WHERE condition one is true
OR condition two is true
OR condition three is true

Which makes more sense to do? since all of the code from the "SELECT from db" statement is the same? It appears that the union condition is more efficient, but my query is VERY long because of it....

Thanks!

Answer 1

LEFT JOIN
joinedTable jt ON jt.someCol = someCol AND jt.someCol = conditions AND @neededjoin ...
...

OR

LEFT JOIN
(
SELECT col1, someCol, col2 FROM joinedTable WHERE someCol = conditions AND @neededjoin ...
) jt ON jt.someCol = someCol
...

OR

;WITH jtCTE AS
(SELECT col1, someCol, col2 FROM joinedTable WHERE someCol = conditions AND @neededjoin ...)
SELECT
...
LEFT JOIN
jtCTE ON jtCTE.someCol = someCol
...

To be honest, there is no such construct as a conditional JOIN unless you use literals.

If it's in the SQL statement it's evaluated... so don't have it in the SQL statement by using dynamic SQL or IF ELSE

Answer 2

the dynamic sql solution is usually the best for these situations, but if you really need to get away from that a series of if statments in a stroed porc will do the job. It's a pain and you have to write much more code but it will be faster than trying to make joins conditional in the statement itself.

Answer 3

I would go for a simple and straightforward approach like this:

DECLARE @ret TABLE(...) ;

IF <coondition one> BEGIN ;
  INSERT INTO @ret() SELECT ...
END ;

IF <coondition two> BEGIN ;
  INSERT INTO @ret() SELECT ...
END ;

IF <coondition three> BEGIN ;
  INSERT INTO @ret() SELECT ...
END ;

SELECT DISTINCT ... FROM @ret ;

Edit: I am suggesting a table variable, not a temporary table, so that the procedure will not recompile every time it runs. Generally speaking, three simpler inserts have a better chance of getting better execution plans than one big huge monster query combining all three.

However, we can not guess-timate performance. we must benchmark to determine it. Yet simpler code chunks are better for readability and maintainability.

Answer 4

The dynamic SQL solution is best in most respects; you are trying to run different queries with different numbers of joins without rewriting the query to do different numbers of joins - and that doesn't work very well in terms of performance.

---

When I was doing this sort of stuff an æon or so ago (say the early 90s), the language I used was I4GL and the queries were built using its CONSTRUCT statement. This was used to generate part of a WHERE clause, so (based on the user input), the filter criteria it generated might look like:

a.column1 BETWEEN 1 AND 50 AND
b.column2 = 'ABCD' AND
c.column3 > 10

In those days, we didn't have the modern JOIN notations; I'm going to have to improvise a bit as we go. Typically there is a core table (or a set of core tables) that are always part of the query; there are also some tables that are optionally part of the query. In the example above, I assume that 'c' is the alias for the main table. The way the code worked would be:

• Note that table 'a' was referenced in the query:
  • Add 'FullTableName AS a' to the FROM clause
  • Add a join condition 'AND a.join1 = c.join1' to the WHERE clause
• Note that table 'b' was referenced...
  • Add bits to the FROM clause and WHERE clause.
• Assemble the SELECT statement from the select-list (usually fixed), the FROM clause and the WHERE clause (occasionally with decorations such as GROUP BY, HAVING or ORDER BY too).

The same basic technique should be applied here - but the details are slightly different.

First of all, you don't have the string to analyze; you know from other circumstances which tables you need to add to your query. So, you still need to design things so that they can be assembled, but...

• The SELECT clause with its select-list is probably fixed. It will identify the tables that must be present in the query because values are pulled from those tables.

• The FROM clause will probably consist of a series of joins.

  • One part will be the core query:

    FROM CoreTable1 AS C1
    JOIN CoreTable2 AS C2
         ON C1.JoinColumn = C2.JoinColumn
    JOIN CoreTable3 AS M
         ON M.PrimaryKey = C1.ForeignKey
    

  • Other tables can be added as necessary:

    JOIN AuxilliaryTable1 AS A
         ON M.ForeignKey1 = A.PrimaryKey
    

  • Or you can specify a full query:

    JOIN (SELECT RelevantColumn1, RelevantColumn2
            FROM AuxilliaryTable1
           WHERE Column1 BETWEEN 1 AND 50) AS A
    

  • In the first case, you have to remember to add the WHERE criterion to the main WHERE clause, and trust the DBMS Optimizer to move the condition into the JOIN table as shown. A good optimizer will do that automatically; a poor one might not. Use query plans to help you determine how able your DBMS is.

• Add the WHERE clause for any inter-table criteria not covered in the joining operations, and any filter criteria based on the core tables. Note that I'm thinking primarily in terms of extra criteria (AND operations) rather than alternative criteria (OR operations), but you can deal with OR too as long as you are careful to parenthesize the expressions sufficiently.

• Occasionally, you may have to add a couple of JOIN conditions to connect a table to the core of the query - that is not dreadfully unusual.

• Add any GROUP BY, HAVING or ORDER BY clauses (or limits, or any other decorations).

Note that you need a good understanding of the database schema and the join conditions. Basically, this is coding in your programming language the way you have to think about constructing the query. As long as you understand this and your schema, there aren't any insuperable problems.

Good luck...

Answer 5

Try this:

LEFT JOIN joinedTable jt
   ON jt.someCol = someCol
   AND jt.someCol = conditions
   AND @neededJoin = 1 -- or whatever indicates join is needed

I think you'll find it is good performance and does what you need.

Update

If this doesn't give the performance I claimed, then perhaps that's because the last time I did this using joins to a table. The value I needed could come from one of 3 tables, based on 2 columns, so I built a 'join-map' table like so:

Col1  Col2 TableCode
  1     2    A
  1     4    A
  1     3    B
  1     5    B
  2     2    C
  2     5    C
  1     11   C

Then,

SELECT
   V.*,
   LookedUpValue =
      CASE M.TableCode
      WHEN 'A' THEN A.Value
      WHEN 'B' THEN B.Value
      WHEN 'C' THEN C.Value
      END
FROM
    ValueMaster V
    INNER JOIN JoinMap M ON V.Col1 = M.oOl1 AND V.Col2 = M.Col2
    LEFT JOIN TableA A ON M.TableCode = 'A'
    LEFT JOIN TableB B ON M.TableCode = 'B'
    LEFT JOIN TableC C ON M.TableCode = 'C'

This gave me a huge performance improvement querying these tables (most of them dozens or hundreds of million-row tables).

This is why I'm asking if you actually get improved performance. Of course it's going to throw a join into the execution plan and assign it some cost, but overall it's going to do a lot less work than some plan that just indiscriminately joins all 3 tables and then Coalesce()s to find the right value.

If you find that compared to dynamic SQL it's only 5% more expensive to do the joins this way, but with the indiscriminate joins is 100% more expensive, it might be worth it to you to do this because of the correctness, clarity, and simplicity over dynamic SQL, all of which are probably more valuable than a small improvement (depending on what you're doing, of course).

Whether the cost scales with the number of rows is also another factor to consider. If even with a huge amount of data you only save 200ms of CPU on a query that isn't run dozens of times a second, it's a no-brainer to use it.

The reason I keep hammering on the fact that I think it's going to perform well is that even with a hash match, it wouldn't have any rows to probe with, or it wouldn't have any rows to create a hash of. The hash operation is going to stop a lot earlier compared to using the WHERE clause OR-style query of your initial post.

Answer 6

Just because no one else mentioned this, here's something that you could use (not dynamic). If the syntax looks weird, it's because I tested it in Oracle.

Basically, you turn your joined tables into sub-selects that have a where clause that returns nothing if your condition does not match. If the condition does match, then the sub-select returns data for that table. The Case statement lets you pick which column is returned in the overall select.

with m as (select 1 Num, 'One' Txt from dual union select 2, 'Two' from dual union select 3, 'Three' from dual),
t1 as (select 1 Num from dual union select 11 from dual),
t2 as (select 2 Num from dual union select 22 from dual),
t3 as (select 3 Num from dual union select 33 from dual)
SELECT m.*
      ,CASE 1
         WHEN 1 THEN
          t1.Num
         WHEN 2 THEN
          t2.Num
         WHEN 3 THEN
          t3.Num
       END SelectedNum
  FROM m
  LEFT JOIN (SELECT * FROM t1 WHERE 1 = 1) t1 ON m.Num = t1.Num
  LEFT JOIN (SELECT * FROM t2 WHERE 1 = 2) t2 ON m.Num = t2.Num
  LEFT JOIN (SELECT * FROM t3 WHERE 1 = 3) t3 ON m.Num = t3.Num