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

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.

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

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.

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.

Each record from the left table will be returned as many times as there are matching records on the right table -- at least 1, but could easily be more than 1.

Could it be a one to many relationship between the left and right tables?

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.

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
)

It seems as though there are multiple rows in the DATA.Dim_Member table per SUSP.Susp_Visits row.

if multiple (x) rows in Dim_Member are associated with a single row in Susp_Visits, there will be x rows in the resul set.

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

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