Should You Filter Before or After a Left Join?

Optimizing left join queries: filtering first or filtering last?

In data analysis and processing, it is often necessary to filter the data in a table before performing a left join to ensure that only the required rows in the left table are included in the join result.

Problem Description

Suppose there are two tables, "Customer" and "Entry". We want to perform a left join based on the "Customer" column, but only include the "Category" column in the "Entry Table" equal to ' D' entry. If you apply the filter using the WHERE clause after the join, matching rows in the left table will be excluded.

Solution

To filter the left table before joining, move the filter condition to the ON clause of the left join statement. This ensures that the left table rows are filtered before being joined with the right table.

<code class="language-sql">SELECT c.Customer, c.State, e.Entry
FROM Customer c
LEFT JOIN Entry e
   ON c.Customer = e.Customer AND e.Category = 'D'</code>

Explanation

In this query, the ON clause contains an additional condition e.Category = 'D' that filters the "entries table" based on the "category" column. Rows from the Customers table will be included in the join only if there is a Customer in the Customers table that matches the specified Category value.

Advantages

Filtering the left table before joining ensures:

• All left table rows will be taken into account regardless of whether there are rows in the left table that match the right table.
• The join result contains only the required rows from both tables.

Conclusion

With this technique, you can effectively filter the data in a table before performing a left join. This ensures that the connection results accurately reflect the data requirements required for the analysis or data processing task.

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