Semi Join

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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.    

Oracle Semi Join

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. /  

Oracle Semi Join 2

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

Oracle Semi Join 3

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.