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OUTER JOIN (U-SQL)

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Summary

An outer join will combine the selected columns from the two joined rowsets for every combination of rows that satisfy the join predicate and will add the rows that are not having a match for the specified join side.

If LEFT is specified (or just OUTER JOIN) then all rows from the left side rowset will be selected and for any rows that have no match in the right rowset, the right rowset columns will receive a null value.

Examples

A. LEFT OUTER JOIN
Using the input rowsets

EmpName DepID
Rafferty 31
Jones 33
Heisenberg 33
Robinson 34
Smith 34
Williams null
DeptID DepName
31 Sales
33 Engineering
34 Clerical
35 Marketing

the following left outer join

@employees = SELECT *  
               FROM (VALUES   
                      ("Rafferty", (int?) 31)  
                    , ("Jones", (int?) 33)  
                    , ("Heisenberg", (int?) 33)  
                    , ("Robinson", (int?) 34)  
                    , ("Smith", (int?) 34)  
                    , ("Williams", (int?) null)) AS E(EmpName, DepID);  
                      
@departments = SELECT *  
                FROM (VALUES  
                       ((int) 31, "Sales")  
                     , ((int) 33, "Engineering")  
                     , ((int) 34, "Clerical")  
                     , ((int) 35, "Marketing")) AS D(DepID, DepName);  
                       
@rs_leftouter =   
    SELECT e.EmpName, d.DepName  
    FROM @employees AS e   
         LEFT OUTER JOIN (SELECT (int?) DepID AS DepID, DepName FROM @departments) AS d  
         ON e.DepID == d.DepID;  
  
OUTPUT @rs_leftouter   
TO "/output/rsLeftOuterJoin.csv"  
USING Outputters.Csv();

produces this rowset

EmpName DepName
Rafferty Sales
Jones Engineering
Heisenberg Engineering
Robinson Clerical
Smith Clerical
Williams null

Since the employee with the name Williams did not have an assigned department, its row receives a null in the DepName column.

If RIGHT is specified then all rows from the right side rowset will be selected and for any rows that have no match in the left rowset, the left rowset columns will receive a null value.

B. RIGHT OUTER JOIN
Using the same input rowsets as above, the following right outer join

@rs_rightouter =   
    SELECT e.EmpName, d.DepName  
    FROM @employees AS e   
         RIGHT OUTER JOIN (SELECT (int?) DepID AS DepID, DepName FROM @departments) AS d  
         ON e.DepID == d.DepID;

produces this rowset

EmpName DepName
Rafferty Sales
Heisenberg Engineering
Jones Engineering
Smith Clerical
Robinson Clerical
null Marketing

Since the Marketing department did not have any assigned employees, its row receives a null in the EmpName column.

If FULL is specified then all rows from both the left and right side rowsets will be selected and for any rows where their match on the other side is missing, the “missing” columns will receive a null value.

C. FULL OUTER JOIN
Using the same input rowsets as above, the following full outer join

@rs_fullouter =   
    SELECT e.EmpName, d.DepName  
    FROM @employees AS e   
         FULL OUTER JOIN (SELECT (int?) DepID AS DepID, DepName FROM @departments) AS d  
         ON e.DepID == d.DepID;

produces this rowset

EmpName DepName
Rafferty Sales
Jones Engineering
Heisenberg Engineering
Robinson Clerical
Smith Clerical
null Marketing
Williams null

In this case both rows that are missing a match are added with null values.

See Also