Anti-join is used to make the queries run faster. It is a very powerful SQL construct Oracle offers for faster queries.
Anti-join between two tables returns rows from the first table where no matches are found in the second table. It is opposite of a semi-join. An anti-join returns one copy of each row in the first table for which no match is found.
Anti-joins are written using the NOT EXISTS or NOT IN constructs.
Let’s take two tables “departments” and “customer”
- 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
- 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 NOT EXISTS
- SELECT 1
- FROM customer
- WHERE customer.department_id = departments.department_id
- ORDER BY departments.department_id;