Скачать или смотреть How to Use a CASE Statement in Snowflake to Handle NULL Values

Discover how to effectively write a `CASE` statement in Snowflake to show transitions between NULL and non-NULL values in your data tables.
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This video is based on the question https://stackoverflow.com/q/76267414/ asked by the user 'Angela B' ( https://stackoverflow.com/u/21909867/ ) and on the answer https://stackoverflow.com/a/76267466/ provided by the user 'SelVazi' ( https://stackoverflow.com/u/4286884/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Displaying Changes in Values Including NULLs

When working with databases, especially in a platform like Snowflake, you may encounter challenges with NULL values. For example, when you have a dataset that includes countries, some entries might contain NULL, signifying missing information.

In this scenario, you want to create a new column that illustrates how values change between two columns (COUNTRY_1 and COUNTRY_2). This includes transitions from NULL to a value and vice versa. Here’s a visual representation of what you might be starting with:

IDCOUNTRY_1COUNTRY_25USACAN6NULLUSA7MEXNULLYou aim to transform this data into a table that highlights changes, such as:

IDCHANGES5USA - CAN6NULL - USA7MEX - NULLHowever, your initial SQL query only accounted for non-null comparisons and didn't display changes involving NULL values.

Crafting a Solution: A Snowflake CASE Statement

To properly handle transitions including NULL values, you can adjust your SQL query by employing a CASE statement. Here's how to do it effectively:

Step 1: Create a Common Table Expression (CTE)

First, you will use a Common Table Expression (CTE) to transform NULL values into a recognizable string, like 'NULL'. This ensures that such values are visible when you are comparing them.

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Explanation of the CTE

The CTE processes the original table MY_TABLE.

It checks each COUNTRY column: if a value is NULL, it replaces it with the string 'NULL'.

Step 2: Write the Query to Display Changes

Next, you will apply the main query that utilizes the newly created CTE for your comparisons:

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Explanation of the Main Query

The SELECT statement grabs data from the CTE.

It uses another CASE statement to compare COUNTRY_1 and COUNTRY_2:

If they are different, it formats the output to show the change as "OLD - NEW".

If they are the same, it returns an empty string.

Resulting Table

The result of this query will give you the required output showcasing all transitions, including those involving NULL.

IDCHANGES5USA - CAN6NULL - USA7MEX - NULLConclusion

By making these adjustments to your SQL queries in Snowflake, you can successfully visualize changes in your data, even when NULL values are involved. This method not only enhances your data analysis but also provides clearer insights into your dataset.

Armed with this knowledge, you now have the tools to investigate your data more thoroughly and communicate findings with greater clarity.