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# Cardinality  and outer join

Posted on 2012-03-21
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what is the best way to join the below.. FULL Outer or LEFT Outer?

Each TableB has Zero or more TableA (Outer Join) (Cardinality 0..n)
Each tableA has one more tableB (Cardinality 1..n)
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Question by:25112

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sdstuber earned 100 total points
ID: 37747235
"best" is irrelevant when comparing those.

they are functionally different.  Use the one that returns the correct results.

If, because of particular data sets, the results are the same, then LEFT "should" be more efficient than "FULL"
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Assisted Solution

ID: 37747244
Create the query so that it makes sense from a business perspective FIRST.  If it then does not perform acceptibly under real life conditions, tune it.  Do not make every query a test in mathematics.

The main rule is to filter first and join second.  The optomizer handles most other considerations and should change what it needs to if the mathematics require it to....
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sachitjain earned 100 total points
ID: 37747252
It totally depends on what do you want out of your query. I take one example.
Employee
EmpId      CityCode
1                 JAI
2                CHN
3                 null
4                 CHN

City
CityCode      CityName
JAI            Jaipur
CHN            Chennai

This query
select EmpId, CityName from Employee left join City on Employee.CityCode = City.CityCode

would give you
EmpId        CityName
1                 Jaipur
2                Chennai
3                 null
4                 Chennai

and following query
select EmpId, CityName from Employee full outer join City on Employee.CityCode = City.CityCode

would give you
EmpId        CityName
1                 Jaipur
2                Chennai
3                 null
4                 Chennai

So query with left join picks everything from entity set that is on left side of join and only selective records from entity set on the right based on join condition. On the other hand, full outer join considers all records from both the entity sets involved in join.
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Author Comment

ID: 37747639
OK.. what is the significance of (Cardinality 0..n) vs (Cardinality 1..n)
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Assisted Solution

ID: 37747846
(Cardinality 1..n)
means that at least one or more row exists (thus you may get one match or one or multiples if it is on the relationship line that you are seeing this designation)

(Cardinality 0..n)
means that zero or more row exists (thus you may get no matches or one or multiples if it is on the relationship line that you are seeing this designation)

Different packages use different terminology. Cardinality generaly applies to uniqueness of keys.
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Accepted Solution

ID: 37747877
LEFT OUTER JOIN means the table on your left can be considered your primary table of interest and you want all rows in that table to be included in your result set regardless of whether there is a match in the table on the right or not.  If there is no match in the table on the right, it will place nulls in the right tables field values...

Full Outer JOIN means join all permutation of keys on both sides.  If no match on the opposing side, nulls will be placed in the fields on the non-matching side.

Does this help clear up the confusion?
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Author Comment

ID: 37780094
thanks
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