How to Find Calls Without Corresponding Phone Book Entries?

Identifying Unmatched Call Records

This guide demonstrates how to find call records in a "Call" table that lack corresponding entries in a "Phone_book" table. The goal is to identify calls from numbers not listed in the phone book. We'll explore several SQL approaches to achieve this.

Method 1: Using NOT IN

This method uses the NOT IN clause to filter out phone numbers present in the "Phone_book" table.

<code class="language-sql">SELECT *
FROM Call
WHERE phone_number NOT IN (SELECT phone_number FROM Phone_book);</code>

This query selects all columns from the "Call" table where the phone_number is not found in the list of phone_numbers from the "Phone_book" table.

Method 2: Employing NOT EXISTS

This approach utilizes a subquery with NOT EXISTS to check for the absence of matching phone numbers.

<code class="language-sql">SELECT *
FROM Call
WHERE NOT EXISTS
  (SELECT 1
   FROM Phone_book
   WHERE Phone_book.phone_number = Call.phone_number);</code>

The subquery searches for a match. If no match is found (NOT EXISTS), the outer query includes the record.

Method 3: Leveraging LEFT OUTER JOIN

This method employs a LEFT OUTER JOIN to combine both tables, then filters for null values in the "Phone_book" table.

<code class="language-sql">SELECT *
FROM Call
LEFT OUTER JOIN Phone_Book
  ON Call.phone_number = Phone_book.phone_number
WHERE Phone_book.phone_number IS NULL;</code>

A LEFT OUTER JOIN returns all rows from the "Call" table. If there's no match in "Phone_book", the corresponding columns in "Phone_book" will be NULL. The WHERE clause filters to show only these unmatched rows. This approach is often preferred for its performance advantages, especially with larger datasets.

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