How Can I Use a LEFT JOIN to Find Records in One Table That Don't Exist in Another?

Use LEFT JOIN to select records in one table that do not exist in another table

When working with multiple tables in a database, determining which records in one table do not exist in another table is a common task. This article will explore how to use SQL queries to accomplish this task.

Suppose there are two tables, table1 and table2, both containing columns named id and name. The goal is to select all names from table2 that do not appear in table1.

Traditional method

The initially provided query attempts to select names not in table1 directly from table2:

SELECT name   
FROM table2  
-- that are not in table1 already

However, this query relies on specific database functionality that may not be universally available.

LEFT JOIN solution

Instead, we can use LEFT JOIN to do this efficiently:

SELECT t1.name
FROM table1 t1
LEFT JOIN table2 t2 ON t2.name = t1.name
WHERE t2.name IS NULL

Explanation

In this query, we join table1 (aliased as t1) and table2 (aliased as t2) by comparing their name column. LEFT JOIN keeps all rows in table1 and returns matching rows in table2. For rows in table1 that have no matching rows in table2, the matching rows in table2 will be NULL.

Finally, we use the WHERE clause to select only the rows in the result that have a matching row of NULL in table2. This ensures that we only select names in table2 that are not present in table1.

Advantages of LEFT JOIN method

LEFT JOIN method has the following advantages:

• Versatility: It can be used in various database engines.
• Efficiency: In most cases, it generates efficient execution plans.
• Clear and easy to read: The query logic is easy to understand.

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