Complex Join

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Complex Join

DonlSimpson

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September 4, 2020 at 7:31 pm

#3786488
I have to tables T1 and T2 that are related by two columns C1 and C2. Both columns are null-able so it's not as simple as:
```
T1 join T2 
   on T1.C1 = T2.C1 
   and T1.C2 = T2.C2
```
So I've come up with this:
```
T1 join T2 
   on (
      T2.C1 = T1.C1 and T1.C2 is null
      )
   or (
      T2.C2 = T1.C2 and T1.C1 is null
      )
   or (
      T2.C1 = T1.C1 and T2.C2 = T1.C2
      )
   )
```
I gotta believe there's an easier way, but it escapes me.
thanx in advance, d
- This topic was modified 3 years, 7 months ago by DonlSimpson.
Don Simpson

I'm not sure about Heisenberg.

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ScottPletcher SSC Guru Points: 100949 More actions · Answer 1

I would think you'd want this:

T1 join T2

on ((T1.C1 = T2.C1) OR (T1.C1 IS NULL AND T2.C1 IS NULL))

and ((T1.C2 = T2.C2) OR (T1.C2 IS NULL AND T2.C2 IS NULL))

SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.

DonlSimpson SSCertifiable Points: 7043 More actions · Answer 2

Unfortunately, either column can be null in one table and not null in the other. When I said "related by," I didn't mean to imply that there's an FK relationship. It's related data used by a variety of reports.

Don Simpson

I'm not sure about Heisenberg.

ScottPletcher SSC Guru Points: 100949 More actions · Answer 3

So you want a NULL in the column to match *every* column on the other table that isn't NULL? I.e., NULL is like a wildcard match?

SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.

DonlSimpson SSCertifiable Points: 7043 More actions · Answer 4

Yes, that would be true.

Don Simpson

I'm not sure about Heisenberg.

ScottPletcher SSC Guru Points: 100949 More actions · Answer 5

Then your original JOIN is fine and likely the cleanest way to do the join.

SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.

Sergiy SSC Guru Points: 110206 More actions · Answer 6

A bit shorter version:

T1 join T2 
   on (
      T2.C1 = T1.C1 OR T1.C1 is null
      )
   AND (
      T2.C2 = T1.C2 OR T1.C2 is null
      )

When you express it this way it becomes obvious that records in T1 with NULL in both C1 and C2 will be joined to every record in T2. Is it what is expected?

_____________
Code for TallyGenerator

DonlSimpson SSCertifiable Points: 7043 More actions · Answer 7

It's not what is expected, and the data condition won't occur. Each column is null-able, but both cannot be null.

Don Simpson

I'm not sure about Heisenberg.

Jeff Moden SSC Guru Points: 1003865 More actions · Answer 8

DonlSimpson wrote:

It's not what is expected, and the data condition won't occur. Each column is null-able, but both cannot be null.

You should post some readily consumable test data and the desired results and let's find out if that's true or not. 😉 Please see the article at the first link in my signature line below.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

DonlSimpson SSCertifiable Points: 7043 More actions · Answer 9

/*
drop table leftTable;
drop table rightTable;
*/
create table leftTable (C1 int, C2 int, C3 varchar(20));
create table rightTable (C1 int, C2 int, C3 varchar(20));

insert into leftTable values (1, 1, 'leftTable 1');
insert into leftTable values (2, 2, 'leftTable 2');
insert into leftTable values (2, 9, 'leftTable 3');
insert into leftTable values (3, 1, 'leftTable 4');
insert into leftTable values (4, null, 'leftTable 5');

insert into rightTable values (1, null, 'rightTable 1'); -- match (leftTable.C1 = rightTable.C1 and rightTable.C2 is null)
insert into rightTable values (null, 1, 'rightTable 2'); -- match x2 (leftTable.C2 = rightTable.C2 and rightTable.C1 is null)
insert into rightTable values (1, 1, 'rightTable 3'); -- match (leftTable.C1 = rightTable.C1 and leftTable.C2 = rightTable.C2)
insert into rightTable values (null, null, 'rightTable 4'); -- no match

insert into rightTable values (1, 2, 'rightTable 5'); -- no match
insert into rightTable values (2, 1, 'rightTable 6'); -- no match
insert into rightTable values (2, null, 'rightTable 7'); -- match x2
insert into rightTable values (4, null, 'rightTable 8'); -- match
insert into rightTable values (4, 1, 'rightTable 9'); -- no match
insert into rightTable values (null, 4, 'rightTable A'); -- no match

commit;

select * 
from leftTable 
join rightTable 
   on (
      leftTable.C1 = rightTable.C1 and rightTable.C2 is null
      )
   or (
      leftTable.C2 = rightTable.C2 and rightTable.C1 is null
      )
   or (
      leftTable.C1 = rightTable.C1 and leftTable.C2 = rightTable.C2
      )
order by leftTable.C1, leftTable.C2, rightTable.C1, rightTable.C2;

Don Simpson

I'm not sure about Heisenberg.

Jeff Moden SSC Guru Points: 1003865 More actions · Answer 10

Thanks for taking the time to post the readily consumable data.

I agree... Sergiy's shortened version doesn't work the same as the code you posted. I've not worked with Boolean Algebra since 1979 and so any other solution is eluding me. Or, maybe it's not and there is no other solution.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Sergiy SSC Guru Points: 110206 More actions · Answer 11

Still there is a shorter version:

select * 
from leftTable  T1
join rightTable T2 
   on (
      T1.C1 = T2.C1 AND (ISNULL(T1.C2, T2.C2) = ISNULL(T2.C2, T1.C2))
      )
   OR (
      T2.C2 = T1.C2 AND (ISNULL(T1.C1, T2.C1) = ISNULL(T2.C1, T1.C1))
      )

Whoo-hoo!

🙂

_____________
Code for TallyGenerator

DonlSimpson SSCertifiable Points: 7043 More actions · Answer 12

At least I know it's not something trivial that I overlooked. 🙂

This reply was modified 3 years, 7 months ago by DonlSimpson.

Don Simpson

I'm not sure about Heisenberg.

ScottPletcher SSC Guru Points: 100949 More actions · Answer 13

You never want to use ISNULL() in a WHERE or JOIN clause. The code may be slightly longer the other way, but it could potentially run much faster.

SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.

Jeff Moden SSC Guru Points: 1003865 More actions · Answer 14

Sergiy wrote:

Still there is a shorter version:

select * 
from leftTable  T1
join rightTable T2 
   on (
      T1.C1 = T2.C1 AND (ISNULL(T1.C2, T2.C2) = ISNULL(T2.C2, T1.C2))
      )
   OR (
      T2.C2 = T1.C2 AND (ISNULL(T1.C1, T2.C1) = ISNULL(T2.C1, T1.C1))
      )

Whoo-hoo!

🙂

I came up with something similar prior to my previous post and rejected it as a solution on my part because of the ISNULL's. It would be interesting to see if the original code or this codes fairs better on a large, properly indexed pair of tables.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)