How to Filter a Table Before a Left Join in SQL?

Filter table before left join

In SQL, join tables are a powerful tool for combining data from multiple data sources. However, you may need to filter one of the tables before performing the join operation.

Suppose you have two tables: Customer and Entry. You want to retrieve all records in the Customers table regardless of whether they have corresponding entries in the Entries table, but only customers with category "D" in the Entries table.

A simple left join query looks like this:

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

However, this query will exclude customers with ID "C" because they don't have any entries with category "D". To ensure that all customer records are included, you can move the filter into the join condition:

<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>

By placing the filter inside the join condition, you can effectively filter the entry table before performing the join. This allows you to retrieve all customers regardless of whether they have matching entries in the entries table, while still only including entries with category "D" in the results.

This solution provides a flexible way to control the data included in the join results, allowing you to perform more precise and optimized data operations.

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