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