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SEMIJOIN (U-SQL)

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Summary

Semijoins are U-SQL’s way to filter a rowset based on the inclusion of its rows in another rowset. Other SQL dialects express this with the SELECT * FROM A WHERE A.key IN (SELECT B.key FROM B) pattern. There are two variants: LEFT SEMIJOIN and RIGHT SEMIJOIN.

A LEFT SEMIJOIN (or just SEMIJOIN) gives only those rows in the left rowset that have a matching row in the right rowset.

The RIGHT SEMIJOIN gives only those rows in the right rowset that have a matching row in the left rowset.

The join expression in the ON clause specifies how to determine the match.

Examples

Given the following 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 query finds all employees that are in valid departments by finding all the employees in the left @employees rowset that have a depID that is listed in the right @departments rowset):

@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);  
                       
@emps_in_valid_dept =  
    SELECT e.EmpName, e.DepID  
    FROM @employees AS e  
         LEFT SEMIJOIN (SELECT (int?) DepID AS DepID, DepName FROM @departments) AS d  
         ON e.DepID == d.DepID;  
  
OUTPUT @emps_in_valid_dept   
TO "/output/rsLeftSemiJoinEmployeesInValidDept.csv"  
USING Outputters.Csv();

The resulting rowset looks like:

EmpName DepID
Rafferty 31
Jones 33
Heisenberg 33
Robinson 34
Smith 34

The following query finds all departments with at least one employee:

@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);  
                       
@depts_with_emps =  
    SELECT d.DepName, d.DepID  
    FROM @employees AS e  
         RIGHT SEMIJOIN (SELECT (int?) DepID AS DepID, DepName FROM @departments) AS d   
         ON e.DepID == d.DepID;  
  
  
OUTPUT @depts_with_emps   
TO "/output/rsRightSemiJoinDepartmentsWithEmployees.csv"  
USING Outputters.Csv();

The query return the rowset:

DepName DepID
Sales 31
Engineering 33
Clerical 34

Additional Example
An example of comparing SEMIJOIN against a subquery with IN is available at Subqueries with IN/NOT IN and SEMIJOIN/ANTISEMIJOIN.

See Also