Sql-server – SQL Server – MERGE join condition with NULLable Columns

mergesql-server-2012

I am trying to merge two of my tables (identical definition).

MERGE INTO xx.dbo.acc_s AS Target 
 USING yy.dbo.acc_s AS Source 
 ON (Target.acc_id= Source.acc_id AND Target.s_id= Source.s_id AND a_code= Source.a_code) 
WHEN NOT matched BY Target THEN
INSERT (acc_id,s_id,a_code)
VALUES (Source.acc_id,Source.s_id,Source.a_code);

What I expected is that "If Target doesn't have the row, INSERT it" – Nothing Else.

I got a "Violation of PRIMARY KEY" error on one of the rows. If it's a matching row BY TARGET, it shouldn't have attempted to insert it in the first place.

What I did after that was:

MERGE INTO xx.dbo.acc_s AS Target 
 USING yy.dbo.acc_s AS Source 
 ON (Target.acc_id= Source.acc_id AND Target.s_id= Source.s_id AND ISNULL(a_code, '')= ISNULL(Source.a_code, '') 
WHEN NOT matched BY Target THEN
INSERT (acc_id,s_id,a_code)
VALUES (Source.acc_id,Source.s_id,Source.a_code);

And it picked up correctly. From this, I am thinking that when I am merging, if I have two NULL columns joining – SQL Server cannot resolve this correctly. Have I encountered an expected behaviour here?

I know that I should probably rather use

WHEN matched THEN
UPDATE SET a_code = Source.a_code

But in that case I might have to change quite a lot of things as I am trying to find a general way to update other tables by creating a template query. However, I understand that it might not be practical.

Best Answer

Mark Sinkinson nailed it in the comments:

This is standard behaviour. NULL does not equal NULL

What you can do is adjust your predicate from this:

WHERE Destination.ColumnA = Source.ColumnA
    AND ...  -- other column comparisons

to the following:

WHERE ((Destination.ColumnA = Source.ColumnA)
       OR (Destination.ColumnA IS NULL AND Source.ColumnA IS NULL))
   AND ...  -- other column comparisons

This will provide a workaround that effectively equates a NULL in one column to a NULL in another.

So to work that into your Merge, it would be as follows:

MERGE INTO xx.dbo.acc_s AS Target 
 USING yy.dbo.acc_s AS Source 
 ON (Target.acc_id= Source.acc_id 
     AND Target.s_id= Source.s_id 
     AND (a_code= Source.a_code 
          OR (a_code IS NULL AND Source.a_code IS NULL)
         )
    ) 
WHEN NOT matched BY Target THEN
INSERT (acc_id,s_id,a_code)
VALUES (Source.acc_id,Source.s_id,Source.a_code);

Related Solutions

Sql-server – How to delete only related records in a multi-key MERGE in SQL Server

This is the separate DELETE operation I had in mind:

DELETE m
FROM dbo.Mapping AS m
WHERE EXISTS 
  (SELECT 1 FROM @Values WHERE LeftID = m.LeftID)
AND NOT EXISTS 
  (SELECT 1 FROM @Values WHERE LeftID = m.LeftID AND RightID = m.RightID);

As I outline here, for a left anti-semi join, the NOT EXISTS pattern will often outperform the LEFT JOIN / NULL pattern (but you should always test).

Not sure if your overall goal is clarity or performance, so only you can judge if this will work out better for your requirements than the NOT MATCHED BY source option. You'll have to look at the plans qualitatively, and the plans and/or runtime metrics quantitatively, to know for sure.

If you expect your MERGE command to protect you from race conditions that would happen with multiple independent statements, you better make sure that is true by changing it to:

MERGE dbo.Mapping WITH (HOLDLOCK) AS target

(From Dan Guzman's blog post.)

Personally, I would do all of this without MERGE, because there are unresolved bugs, among other reasons. And Paul White seems to recommend separate DML statements as well.

And here's why I added a schema prefix: you should always reference objects by schema, when creating, affecting, etc.

Mysql – 3 table merge with some condition

As first step I merge the two product tables. I need an full outer join on product1 and product2 tables. To obtain on outer join in mysql we can union a left join and a right join. Start with a left join. In the following query IFNULL(p1.key1, p2.key1) is useful to merge two key column coming from the two product tables and IFNULL(, 'NaN') is useful to obtain 'NaN' in the output.

select IFNULL(p1.key1, p2.key1) as key1, 
       IFNULL(p1.key2, p2.key2) as key2,
       IFNULL(p1.serial, p2.serial) as serial,
       IFNULL(p1.product_data, 'NaN') as product_data1, 
       IFNULL(p2.product_data, 'NaN') as product_data2
from       product1 p1
left join  product2 p2 on p1.key1 = p2.key1 and p1.key2 = p2.key2 and p1.serial =     p2.serial;

To obtain a full outer join I need to repeat the previous query with 'right join' and union the result with the 'left join'

select IFNULL(p1.key1, p2.key1) as key1, 
       IFNULL(p1.key2, p2.key2) as key2,
       IFNULL(p1.serial, p2.serial) as serial,
       IFNULL(p1.product_data, 'NaN') as product_data1, 
       IFNULL(p2.product_data, 'NaN') as product_data2
from       product1 p1
left join  product2 p2 on p1.key1 = p2.key1 and p1.key2 = p2.key2 and p1.serial = p2.serial
union
select IFNULL(p1.key1, p2.key1) as key1, 
       IFNULL(p1.key2, p2.key2) as key2,
       IFNULL(p1.serial, p2.serial) as serial,
       IFNULL(p1.product_data, 'NaN') as product_data1, 
       IFNULL(p2.product_data, 'NaN') as product_data2
from       product1 p1
right join  product2 p2 on p1.key1 = p2.key1 and p1.key2 = p2.key2 and p1.serial = p2.serial;

For convenience, I create a view with the previous query:

create or replace view p12 as
select IFNULL(p1.key1, p2.key1) as key1, 
       IFNULL(p1.key2, p2.key2) as key2,
       IFNULL(p1.serial, p2.serial) as serial,
       IFNULL(p1.product_data, 'NaN') as product_data1, 
       IFNULL(p2.product_data, 'NaN') as product_data2
from       product1 p1
left join  product2 p2 on p1.key1 = p2.key1 and p1.key2 = p2.key2 and p1.serial = p2.serial
union
select IFNULL(p1.key1, p2.key1) as key1, 
       IFNULL(p1.key2, p2.key2) as key2,
       IFNULL(p1.serial, p2.serial) as serial,
       IFNULL(p1.product_data, 'NaN') as product_data1, 
       IFNULL(p2.product_data, 'NaN') as product_data2
from       product1 p1
right join  product2 p2 on p1.key1 = p2.key1 and p1.key2 = p2.key2 and p1.serial = p2.serial;

To check we can try with:

mysql> select * from p12 where key1 = 1 and key2 =1 ;
+------+------+--------+---------------+---------------+
| key1 | key2 | serial | product_data1 | product_data2 |
+------+------+--------+---------------+---------------+
| 1    | 1    | 0      | 15.556        | 5.556         |
| 1    | 1    | 1      | 14.996        | 4.996         |
| 1    | 1    | 2      | 12.556        | NaN           |
| 1    | 1    | 3      | 15.669        | NaN           |
+------+------+--------+---------------+---------------+

So the he final query is:

select mi.key1, mi.key2, p12.serial, mi.info1, mi.info2, mi.date, p12.product_data1, p12.product_data2 
from main_info mi 
inner join p12 on mi.key1 = p12.key1 and mi.key2 = p12.key2
order by mi.key1, mi.key2, p12.serial;


+------+------+--------+-------+-------+----------+---------------+---------------+
| key1 | key2 | serial | info1 | info2 | date     | product_data1 | product_data2 |
+------+------+--------+-------+-------+----------+---------------+---------------+
| 1    | 1    | 0      | 15    | 90    | 20120501 | 15.556        | 5.556         |
| 1    | 1    | 1      | 15    | 90    | 20120501 | 14.996        | 4.996         |
| 1    | 1    | 2      | 15    | 90    | 20120501 | 12.556        | NaN           |
| 1    | 1    | 3      | 15    | 90    | 20120501 | 15.669        | NaN           |
| 1    | 2    | 0      | 14    | 92    | 20120601 | 12.556        | 2.556         |
| 1    | 2    | 1      | 14    | 92    | 20120601 | 13.335        | 3.335         |
| 1    | 2    | 2      | 14    | 92    | 20120601 | NaN           | 2.56          |
| 1    | 2    | 3      | 14    | 92    | 20120601 | NaN           | 3.556         |
| 1    | 3    | 1      | 15    | 82    | 20120801 | 12.225        | 2.225         |
| 1    | 3    | 2      | 15    | 82    | 20120801 | 13.556        | 3.556         |
| 1    | 3    | 3      | 15    | 82    | 20120801 | 14.556        | NaN           |
| 2    | 1    | 0      | 17    | 90    | 20130302 | 12.556        | NaN           |
| 2    | 1    | 1      | 17    | 90    | 20130302 | 13.553        | NaN           |
| 2    | 1    | 2      | 17    | 90    | 20130302 | 12.335        | NaN           |
| 2    | 2    | 0      | 16    | 88    | 20130601 | NaN           | 2.556         |
| 2    | 2    | 1      | 16    | 88    | 20130601 | NaN           | 3.553         |
+------+------+--------+-------+-------+----------+---------------+---------------+

Best Answer

Related Solutions

Sql-server – How to delete only related records in a multi-key MERGE in SQL Server

Mysql – 3 table merge with some condition

Related Question