Oracle Semi Join

Semi-join is introduced in Oracle 8.0. It provides an efficient method of performing a WHERE EXISTS sub-query.

A semi-join returns one copy of each row in first table for which at least one match is found.

Semi-joins are written using the EXISTS construct.

Oracle Semi Join Example

Let’s take two tables “departments” and “customer”

Departments table

1. CREATE TABLE  “DEPARTMENTS”   
2.    (    “DEPARTMENT_ID” NUMBER(10,0) NOT NULL ENABLE,   
3.     “DEPARTMENT_NAME” VARCHAR2(50) NOT NULL ENABLE,   
4.      CONSTRAINT “DEPARTMENTS_PK” PRIMARY KEY (“DEPARTMENT_ID”) ENABLE  
5.    )  
6. /  
7.    

Customer table

1. CREATE TABLE  “CUSTOMER”   
2.    (    “CUSTOMER_ID” NUMBER,   
3.     “FIRST_NAME” VARCHAR2(4000),   
4.     “LAST_NAME” VARCHAR2(4000),   
5.     “DEPARTMENT_ID” NUMBER  
6.    )  
7. /  

Execute this query

1. SELECT   departments.department_id, departments.department_name  
2.         FROM     departments  
3.         WHERE    EXISTS  
4.                  (  
5.                  SELECT 1  
6.                  FROM   customer  
7.                  WHERE customer.department_id = departments.department_id  
8.                  )  
9.         ORDER BY departments.department_id;  

Output

Difference between anti-join and semi-join

While a semi-join returns one copy of each row in the first table for which at least one match is found, an anti-join returns one copy of each row in the first table for which no match is found.