CODEWITHSUNDEEP
sqlsql-server
John Bustos
John Bustos

Reputation: 19574

SQL Server JOIN missing NULL values

Suppose I had the following 2 tables:

      Table1:                                Table2:
Col1:      Col2:     Col3:             Col1:       Col2:       Col4:
a          b         c                 a           b           d
e          <null>    f                 e           <null>      g
h          i         j                 h           i           k
l          <null>    m                 l           <null>      n
o          <null>    p                 o           <null>      q

Now, I want to join these tables on Col1 and Col2 and bring back the entire set to look like:

     Result:
Col1:      Col2:     Col3:     Col4:
a          b         c         d
e          <null>    f         g
h          i         j         k
l          <null>    m         n
o          <null>    p         q

So, I tried a SQL like:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1
INNER JOIN Table2
    ON Table1.Col1 = Table2.Col1 
    AND Table1.Col2 = Table2.Col2

But it isn't matching the NULL values in Col2, so I end up with:

     Result:
Col1:      Col2:     Col3:     Col4:
a          b         c         d
h          i         j         k

How can I get the result I am looking for??

Thanks!

Upvotes: 103

Views: 248455

Answers (10)

Dave Hackett
Dave Hackett

Reputation: 587

Use Left Outer Join instead of Inner Join to include rows with NULLS.

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1
LEFT OUTER JOIN Table2 
    ON Table1.Col1 = Table2.Col1 
    AND Table1.Col2 = Table2.Col2

For more information, see here: http://technet.microsoft.com/en-us/library/ms190409(v=sql.105).aspx

Upvotes: 53

Fabich
Fabich

Reputation: 3069

Some SQL implementations have a special Null-safe equal operator.

For instance Snowflake has EQUAL_NULL so ou can do

SELECT 
  Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4 
FROM 
  Table1 
  INNER JOIN Table2 ON EQUAL_NULL(Table1.Col1, Table2.Col1) 
  AND EQUAL_NULL(Table1.Col2, Table2.Col2)

Upvotes: 0

ListenSoftware Louise Ai Agent
ListenSoftware Louise Ai Agent

Reputation: 4263

declare @Table1 as Table(Col1 varchar(1),Col2 varchar(1), Col3 varchar(1))
declare @Table2 as Table(Col1 varchar(1),Col2 varchar(1), Col4 varchar(1))
insert into @Table1
values('a',          'b',     'c'),
('e',          null,    'f'), 
('h',          'i'  ,   'j'), 
('l',          null  ,  'm'), 
('o',          null  ,  'p') 
insert into @Table2
values('a',          'b',     'd'),
('e',          null,    'g'), 
('h',          'i'  ,   'k'), 
('l',          null  ,  'n'), 
('o',          null  ,  'q') 


select * from @Table1 tbl1
join @Table2 tbl2
on tbl1.Col1 =tbl2.Col1
and isnull(tbl1.Col2,0) =isnull(tbl2.Col2,0)

output:

 Col1   Col2    Col3    Col1    Col2    Col4
 a      b       c       a       b       d
 e      NULL    f       e       NULL    g
 h      i       j       h       i       k
 l      NULL    m       l       NULL    n
 o      NULL    p       o       NULL    q

Upvotes: 1

Marcel Pfeifer
Marcel Pfeifer

Reputation: 31

The only correct answer is not to join columns with null values. This can lead to unwanted behaviour very quickly.

e.g. isnull(b.colId,''): What happens if you have empty strings in your data? The join maybe duplicate rows which I guess is not intended in this case.

Upvotes: 0

Danny Costin
Danny Costin

Reputation: 11

Try using additional condition in join:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1 
INNER JOIN Table2
ON (Table1.Col1 = Table2.Col1 
    OR (Table1.Col1 IS NULL AND Table2.Col1 IS NULL)
   )

Upvotes: 0

user11333280
user11333280

Reputation: 11

for some reason I couldn't get it to work with the outer join.

So I used:

SELECT * from t1 where not Id in (SELECT DISTINCT t2.id from t2)

Upvotes: 1

Gordon Linoff
Gordon Linoff

Reputation: 1271151

You can be explicit about the joins:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1 INNER JOIN
     Table2
      ON (Table1.Col1 = Table2.Col1 or Table1.Col1 is NULL and Table2.Col1 is NULL) AND
         (Table1.Col2 = Table2.Col2 or Table1.Col2 is NULL and Table2.Col2 is NULL)

In practice, I would be more likely to use coalesce() in the join condition:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1 INNER JOIN
     Table2
     ON (coalesce(Table1.Col1, '') = coalesce(Table2.Col1, '')) AND
        (coalesce(Table1.Col2, '') = coalesce(Table2.Col2, ''))

Where '' would be a value not in either of the tables.

Just a word of caution. In most databases, using any of these constructs prevents the use of indexes.

Upvotes: 149

wiretext
wiretext

Reputation: 3352

you can just map like that

select * from tableA a
join tableB b on isnull(a.colID,'') = isnull(b.colId,'')

Upvotes: 1

sgeddes
sgeddes

Reputation: 62861

Try using ISNULL function:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1 
INNER JOIN Table2
   ON Table1.Col1 = Table2.Col1 
   AND ISNULL(Table1.Col2, 'ZZZZ') = ISNULL(Table2.Col2,'ZZZZ')

Where 'ZZZZ' is some arbitrary value never in the table.

Upvotes: 18

jap1968
jap1968

Reputation: 7763

Dirty and quick hack:

SELECT Table1.Col1, Table1.Col2, Table1.Col3, Table2.Col4
FROM Table1 INNER JOIN Table2 ON Table1.Col1 = Table2.Col1
 AND ((Table1.Col2 = Table2.Col2) OR (Table1.Col2 IS NULL AND Table2.Col2 IS NULL))

Upvotes: 18

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