How can a LEFT OUTER JOIN return more records than exist in the left table?

Question

I have a very basic LEFT OUTER JOIN to return all results from the left table and some additional information from a much bigger table. The left table contains 4935 records yet when I LEFT OUTER JOIN it to an additional table the record count is significantly larger.

As far as I'm aware it is absolute gospel that a LEFT OUTER JOIN will return all records from the left table with matched records from the right table and null values for any rows which cannot be matched, as such it's my understanding that it should be impossible to return more rows than exist in the left table, but it's happening all the same!

SQL Query follows:

SELECT     SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID
FROM         SUSP.Susp_Visits LEFT OUTER JOIN
                      DATA.Dim_Member ON SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum

Perhaps I have made a mistake in the syntax or my understanding of LEFT OUTER JOIN is incomplete, hopefully someone can explain how this could be occurring?

Answer 1

The LEFT OUTER JOIN will return all records from the LEFT table joined with the RIGHT table where possible.

If there are matches though, it will still return all rows that match, therefore, one row in LEFT that matches two rows in RIGHT will return as two ROWS, just like an INNER JOIN.

EDIT: In response to your edit, I've just had a further look at your query and it looks like you are only returning data from the LEFT table. Therefore, if you only want data from the LEFT table, and you only want one row returned for each row in the LEFT table, then you have no need to perform a JOIN at all and can just do a SELECT directly from the LEFT table.

Answer 2

Table1                Table2
_______               _________
1                      2
2                      2
3                      5
4                      6

SELECT Table1.Id, Table2.Id FROM Table1 LEFT OUTER JOIN Table2 ON Table1.Id=Table2.Id

Results:

1,null
2,2
2,2
3,null
4,null

Answer 3

It isn't impossible. The number of records in the left table is the minimum number of records it will return. If the right table has two records that match to one record in the left table, it will return two records.

Answer 4

In response to your postscript, that depends on what you would like.

You are getting (possible) multiple rows for each row in your left table because there are multiple matches for the join condition. If you want your total results to have the same number of rows as there is in the left part of the query you need to make sure your join conditions cause a 1-to-1 match.

Alternatively, depending on what you actually want you can use aggregate functions (if for example you just want a string from the right part you could generate a column that is a comma delimited string of the right side results for that left row.

If you are only looking at 1 or 2 columns from the outer join you might consider using a scalar subquery since you will be guaranteed 1 result.

Answer 5

左表中的每条记录将返回与右表中匹配记录一样多的次数——至少为 1，但很容易超过 1。

Answer 6

左右表之间可能是一对多的关系吗？

Answer 7

LEFT OUTER JOIN just like INNER JOIN (normal join) will return as many results for each row in left table as many matches it finds in the right table. Hence you can have a lot of results - up to N x M, where N is number of rows in left table and M is number of rows in right table.

It's the minimum number of results is always guaranteed in LEFT OUTER JOIN to be at least N.

Answer 8

If you need just any one row from the right side

SELECT SuspReason, SiteID FROM(
    SELECT SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID, ROW_NUMBER()
    OVER(PARTITION BY SUSP.Susp_Visits.SiteID) AS rn
    FROM SUSP.Susp_Visits
    LEFT OUTER JOIN DATA.Dim_Member ON SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum
) AS t
WHERE rn=1

or just

SELECT SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID
FROM SUSP.Susp_Visits WHERE EXISTS(
    SELECT DATA.Dim_Member WHERE SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum
)

Answer 9

请注意，如果在包含左外连接的查询的“右侧”表上有 where 子句...如果右侧没有满足 where 子句的记录，则“左侧”对应的记录' 表不会出现在您的查询结果中....

Answer 10

似乎每个 SUSP.Susp_Visits 行的 DATA.Dim_Member 表中有多行。

Answer 11

如果 Dim_Member 中的多个 (x) 行与 Susp_Visits 中的单个行相关联，则结果集中将有 x 行。

Answer 12

Since the left table contains 4935 records, I suspect you want your results to return 4935 records. Try this:

create table table1
(siteID int, 
SuspReason int)

create table table2
(siteID int, 
SuspReason int)

insert into table1(siteID, SuspReason) values 
(1, 678), 
(1, 186), 
(1, 723)
    
insert into table2(siteID, SuspReason) values 
(1, 678),
(1, 965)
   
select distinct t1.siteID, t1.SuspReason
from table1 t1 left join table2 t2 on t1.siteID = t2.siteID and t1.SuspReason = t2.SuspReason

union 

select distinct t2.siteID, t2.SuspReason 
from table1 t1 right join table2 t2 on t1.siteID = t2.siteID and t1.SuspReason = t2.SuspReason

Answer 13

The only way your query would return more number of rows than the left table ( which is SUSP.Susp_Visits in your case), is that the condition ( SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum ) is matching multiple rows in the right table, which is DATA.Dim_Member . So, there are multiple rows in the DATA.Dim_Member where identical values are present for DATA.Dim_Member.MembershipNum . You can verify this by executing the below query:

select DATA.Dim_Member.MembershipNum, count(DATA.Dim_Member.MembershipNum) from DATA.Dim_Member group by DATA.Dim_Member.MembershipNum

Answer 14

Simply, LEFT OUTER JOIN is the Cartesian product within each join key, along with the unmatched rows of the left table (ie for each key_x that has N records in table_L and M records in table_R the result will have N*M records if M>0, or N records if M=0)