How to Query Two Tables For Duplicate Values in SQL?

When working with relational databases, it's common to identify duplicate values across multiple tables. SQL provides efficient ways to query such data using different techniques. These methods help streamline data analysis and ensure data consistency.

In this article, we demonstrate how to query two tables for duplicate values using two methods such as the INNER JOIN and the WHERE clause. These methods help filter records based on matching conditions, providing a streamlined way to identify duplicates in relational data.

Query Two Tables For Duplicate Values

We have two tables named 'demo_table1' and 'demo_table2' in our geek’s database. These tables contain data with overlapping and distinct records.
Our goal is to identify duplicates based on matching conditions using SQL queries.

demo_table1:

demo_table2:

Using INNER JOIN

The INNER JOIN in SQL is used to combine rows from two or more tables based on a related column between them. It returns only the rows where there is a match in both tables, effectively filtering out rows that do not meet the join condition. This is one of the most commonly used joins for querying relational data with relationships between tables.

Query:

SELECT demo_table1.ID
FROM demo_table1
INNER JOIN demo_table2
ON demo_table1.ID=demo_table2.ID;

Output

Using WHERE Clause

The `WHERE` clause in SQL is used to filter records and specify conditions that must be met for rows to be included in the result set. It allows us to retrieve only the records that match specific criteria, making it essential for narrowing down query results. The `WHERE` clause can work with various operators like `=`, `<`, `>`, `LIKE`, and logical operators like `AND`, `OR`, and `NOT` to build complex filtering conditions.

Query:

SELECT demo_table1.ID
FROM demo_table1, demo_table2
WHERE demo_table1.ID=demo_table2.ID;

Output:

Conclusion

When working with relational databases, it's common to identify duplicate values across multiple tables. SQL provides efficient ways to query such data using different techniques. In this guide, we demonstrate how to query two tables for duplicate values using two methods: the INNER JOIN and the WHERE clause. These methods help filter records based on matching conditions, providing a streamlined way to identify duplicates in relational data.