Oracle EXISTS

Introduction

Oracle EXISTS is a subquery that returns a Boolean value (TRUE or FALSE) depending on whether or not a given subquery returns any rows.

The EXISTS operator is often used in conjunction with the WHERE clause in a SELECT statement to determine whether a particular condition holds true for at least one row in a table.

Syntax

SELECT column_name(s)
FROM table_name
WHERE EXISTS (subquery);

• column_name(s): the name(s) of the column(s) you want to select from the table
• table_name: the name of the table that you want to select data from
• subquery: the subquery that you want to evaluate using the EXISTS operator

Example

For example, let's say we have a table called customers with columns id, name, age, and city. We want to retrieve all customers from the table who are over 18 years old and live in New York City. We can do that with the following query:

SELECT *
FROM customers
WHERE age > 18
AND city = 'New York City'

Now, let's say we only want to know whether at least one customer from New York City is over 18 years old. In this case, we can use the EXISTS operator to evaluate a subquery that checks the same condition:

SELECT EXISTS (
  SELECT *
  FROM customers
  WHERE age > 18
  AND city = 'New York City'
);

If the subquery returns at least one row, the EXISTS operator will return TRUE, and otherwise, it will return FALSE.

Conclusion

The EXISTS operator is a useful tool to determine the presence of data based on a given condition. It allows programmers to perform advanced filtering on data without using complex SQL statements.