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

Concatenating row values from table to single string using T-SQL

There are several ways to concatenate row values into a single string in SQL Server, including using custom SQL functions, COALESCE, or the FOR XML PATH clause. In this post, I will demonstrate how to perform string aggregation using XML PATH and COALESCE. To make these examples easy to follow, I'll be using the NORTHWND sample database to show these methods in action.

Example 1 : COALESCE (find more about COALESCE)

Using COALESCE function , we can concatenate many row values in to one single string. this is one use of COALESCE , there are many ,so you can find some more clicking above mention link.

Example 2: XML PATH

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To ensure our final string doesn't start with an extra comma, we use the STUFF function. STUFF works by deleting a specific portion of a string and replacing it with another. In this case, we use it to target the very first character—the extra comma—and replace it with nothing, effectively removing it from the beginning of our result.