ISNULL vs COALESCE

Feature	ISNULL	COALESCE
Origin	T-SQL Specific (Microsoft)	ANSI SQL Standard (Universal)
Parameters	Accepts only 2 arguments.	Accepts 2 or more arguments.
Data Type	Uses the type of the first argument.	Uses the type with the highest precedence.
Performance	Slightly faster for simple 1-to-1 checks.	Can be slightly slower in complex subqueries.
Evaluation	Evaluates the first argument once.	May evaluate the expression multiple times.

Key Differences Explained

1. Number of Arguments

• ISNULL: Limited to checking one value and providing one replacement.

                SELECT ISNULL(Price, 0) FROM Products;

• COALESCE: Can check multiple columns/values in order and returns the first non-null one.

                SELECT COALESCE(Mobile, HomePhone, OfficePhone, 'N/A') FROM Contacts;

2. Data Type Handling

This is often where developers run into unexpected results or errors.

• ISNULL is rigid. It looks at the data type of the first expression and tries to convert the replacement to match it. If the replacement is longer than the first argument, it may even truncate the data.
• COALESCE follows "Data Type Precedence." It looks at all arguments and converts everything to the type that ranks highest.

3. Performance in SQL Server

In most standard queries, the performance difference is negligible. However:

• ISNULL is built directly into the SQL Server engine as a function.
• COALESCE is actually translated by the engine into a CASE statement.
• Best Practice: Use ISNULL for high-frequency, simple null-to-zero conversions where performance is critical. Use COALESCE for multi-column logic or when writing code that needs to be portable to other databases like PostgreSQL or MySQL.

Note :

Use ISNULL when you are sure you only have two values and want to force a specific data type. Use COALESCE when you need flexibility, multiple fallbacks, or are following ANSI standards