Replacing NULL with 0 in a SQL server query

Diving deep into ISNULL and COALESCE

The art of using ISNULL

ISNULL takes two parameters—the column name and the NULL substitute. It's evaluated once per SELECT clause leading to better performance in certain situations.

-- I promise, no nulls here just Zero Heroes
SELECT ISNULL(column_name, 0) FROM table_name;
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The versatility of COALESCE

COALESCE, unlike ISNULL, gobbles up multiple parameters and spits out the first non-null in line. This can add a lot of flexibility to your code.

-- Knock, Knock. Who's there? Not NULL, buddy.
SELECT COALESCE(column_one, column_two, 0) FROM table_name;
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Be careful though, COALESCE may introduce a slight performance overhead due to multiple evaluations.

Fine-tuning performance

ISNULL can be a tad faster when dealing with massive datasets because it's evaluated once. On the other hand, COALESCE goes through each parameter, which could cause a potential performance hit.

Shunning NULL in aggregate functions

Execute ISNULL within SUM() to prevent NULL from shoving your calculations off course. Replacing NULL with 0 assures unadulterated results.

-- Let's SUM up our totals, NULL not invited
SELECT SUM(ISNULL(column_name, 0)) FROM table_name;
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PUTting Zero in NULL places - directly in tables

Sometimes, it's more straightforward to update your tables directly, padding NULL fields with 0. This tactic would free you from using ISNULL or COALESCE in subsequent queries:

-- NULL, you're evicted. Enter Zero
UPDATE table_name
SET column_name = ISNULL(column_name, 0);
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Playing with CASE for different run statuses

When dealing with various run statuses in your dataset, using CASE facilitates conditional counts:

-- Ready, steady, go - but wait, where's NULL?
SELECT CASE 
         WHEN run_status IS NULL THEN 0
         ELSE run_status
       END
FROM runs;
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This method offers a customized approach to replacing NULL with 0.

Data types matter

Always ensure you're clear about data types. Mismatched data types can yield unanticipated errors or results when using ISNULL or COALESCE.