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Learn how to effectively retrieve maximum values from multiple tables in PostgreSQL without returning null when some tables are empty.
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How to Resolve NULL in PostgreSQL Cross Join with Max Function

Working with SQL can sometimes lead to unexpected results, especially when dealing with joins and aggregate functions. One common issue users face with PostgreSQL is when using the max() function along with a cross join. If even one of the tables being joined has no rows, the entire query may return NULL values for all tables. In this guide, we will delve into the problem and provide clear solutions to ensure you can retrieve the correct values from your tables.

The Problem: NULL Return Values

Consider this SQL query intended to select the maximum values from the modified_at column across three tables (a, b, and c):

[[See Video to Reveal this Text or Code Snippet]]

Why Does This Happen?

The problem arises when one or more of the tables (a, b, or c) are empty (i.e., contain zero rows). In such cases, PostgreSQL performs a cross join, which effectively results in no rows being returned, leading to the output:

[[See Video to Reveal this Text or Code Snippet]]

This behavior can be frustrating when you expect to see values from the tables that do contain data.

The Solution: Using Outer Joins or Subqueries

To get around this issue, you have a couple of effective solutions which we will explore below:

Method 1: Utilize Outer Joins

One approach to ensure you get the maximum values without returning NULL is to use OUTER JOINs. This allows you to join the tables while still keeping the rows that exist in at least one of the tables.

Here's how you can modify your original query:

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Advantages of Using Outer Joins

This method ensures that even if one or more tables have no rows, it still retrieves the maximum values from the tables that do.

You will get separate NULL values for the tables that are empty, rather than a complete output of NULL.

Method 2: Use Subqueries

Another potentially simpler approach is to use subqueries for each table. This keeps each maximum query separate and independent, which allows PostgreSQL to return values correctly without being affected by the empty tables.

Here’s how to implement this method:

[[See Video to Reveal this Text or Code Snippet]]

Advantages of Using Subqueries

Clarity: Each maximum value is clearly derived from its respective table, making the query easier to understand.

Isolation: Each subquery operates independently, so the absence of rows in one doesn't affect the others.

Conclusion

By employing either OUTER JOINs or using subqueries, you can elegantly handle the scenario in PostgreSQL where one or more tables might return zero rows. These methods ensure that you can still extract the maximum values from the tables that contain data, while avoiding the frustrating pitfall of returning NULL for all.

Now you can confidently write your SQL queries without the fear of encountering unexpected NULL outputs. Happy querying!