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

CREATE TABLE  "DEPARTMENTS"   
   (    "DEPARTMENT_ID" NUMBER(10,0) NOT NULL ENABLE,   
    "DEPARTMENT_NAME" VARCHAR2(50) NOT NULL ENABLE,   
     CONSTRAINT "DEPARTMENTS_PK" PRIMARY KEY ("DEPARTMENT_ID") ENABLE  
   )  
/  
   

Customer table

CREATE TABLE  "CUSTOMER"   
   (    "CUSTOMER_ID" NUMBER,   
    "FIRST_NAME" VARCHAR2(4000),   
    "LAST_NAME" VARCHAR2(4000),   
    "DEPARTMENT_ID" NUMBER  
   )  
/  

Execute this query

SELECT   departments.department_id, departments.department_name  
        FROM     departments  
        WHERE    EXISTS  
                 (  
                 SELECT 1  
                 FROM   customer  
                 WHERE customer.department_id = departments.department_id  
                 )  
        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.