What is Semi Join?
Semi-Join matches the rows of two relations and then shows the matching rows of the relation whose name is mentioned on the left side of ⋉ the Join operator. I am describing more details in the below examples. Hopefully, it will help you to understand the semi-join.
Example 1 of Semi Join
| ID | Rank | Salary |
| 101 | Assistant Professor | 80,000 |
| 102 | Associate Professor | 90,000 |
| 103 | Lecturer | 70,000 |
Table 1: Teacher
| ID | RollNo | Marks |
| 103 | 2017 – 01 | 80 |
| 104 | 2017 – 02 | 90 |
| 105 | 2017 – 03 | 70 |
Table 2: Student
| ID | RollNo | Marks |
| 103 | 2017 – 01 | 80 |
Table 3: Student ⋉ Teacher OR Student Semi-Join Teacher
| ID | Rank | Salary |
| 103 | Lecturer | 70,000 |
Table 4: Teacher ⋉ Student OR Teacher SEMI-JOIN Student
Comparison of semi-join with other joins
| Type | Syntax | Syntax | What will do? |
| Semi Join | SELECT * FROM table1 WHERE EXISTS (SELECT 1 FROM table2 WHERE table1.column = table2.column); | SELECT * FROM Student_Table WHERE EXISTS (SELECT 1 FROM Teacher_Table WHERE Student_Table.ClassID = Teacher_Table.ClassID); | Show only the rows from the left table where there is a match in the right table. |
| Cross Join | SELECT * FROM table1 CROSS JOIN table2; | SELECT * FROM Student_Table CROSS JOIN Teacher_Table; | Show the Cartesian product of the two tables, resulting in all possible combinations of rows. |
| Full (Outer) Join | SELECT * FROM table1 FULL JOIN table2 ON table1.column = table2.column; | SELECT * FROM Student_Table FULL JOIN Teacher_Table ON Student_Table.ClassID = Teacher_Table.ClassID; | Show all rows when there is a match in either the left or right table. |
| Inner Join | SELECT * FROM table1 INNER JOIN table2 ON table1.column = table2.column; | SELECT * FROM Student_Table INNER JOIN Teacher_Table ON Student_Table.ClassID = Teacher_Table.ClassID; | Show only the rows where there is a match in both tables based on the specified condition. |
| Left (Outer) Join | SELECT * FROM table1 LEFT JOIN table2 ON table1.column = table2.column; | SELECT * FROM Student_Table LEFT JOIN Teacher_Table ON Student_Table.ClassID = Teacher_Table.ClassID; | Show all rows from the left table and the matched rows from the right table. |
| Right (Outer) Join | SELECT * FROM table1 RIGHT JOIN table2 ON table1.column = table2.column; | SELECT * FROM Student_Table RIGHT JOIN Teacher_Table ON Student_Table.ClassID = Teacher_Table.ClassID; | Show all rows from the right table and the matched rows from the left table. |
| Self Join | SELECT * FROM table1 t1 INNER JOIN table1 t2 ON t1.column = t2.column; | SELECT * FROM Student_Table t1 INNER JOIN Student_Table t2 ON t1.ClassID = t2.ClassID; | Joins a table with itself, treating it as if it were two separate tables. |
Example 2 of Semi Join
| ID | Rank | Salary |
| 101 | Assistant Professor | 80,000 |
| 102 | Associate Professor | 90,000 |
| 103 | Lecturer | 70,000 |
Table 5: Teacher
| ID | RollNo | Marks |
| 101 | 2017 – 01 | 80 |
| 103 | 2017 – 02 | 90 |
| 106 | 2017 – 03 | 70 |
Table 6: Student
| ID | RollNo | Marks |
| 101 | 2017 – 01 | 80 |
| 103 | 2017 – 02 | 90 |
Table 7: Student ⋉ Teacher OR Student SEMI-JOIN Teacher:
| ID | Rank | Salary |
| 101 | Assistant Professor | 80,000 |
| 103 | Lecturer | 70,000 |
Table 8: Teacher ⋉ Student OR Teacher SEMI-JOIN Student
Example 3 of Semi Join
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Topic Covered
Semi-Join with Examples in Relational Algebra and database Systems.