How can I replace the following query with ISNULL function in SQL? I have the following query. For performance optimization reasons I have been asked to replace the logic with ISNULL function in SQL. How can I achieve this? please advice.

                   SELECT CASE 
                               WHEN tb1.[tag-value] IS NOT NULL THEN tb1.[tag-value]
                               ELSE 'N/A'
                          END     AS [tag-value]
                   FROM   @AgentTableFilt ta
This should do what you need

SELECT ISNULL(ta.[tag-value], 'N/A') AS [tag-value]

FROM   @AgentTableFilt ta


Or you could use the following

SELECT COALESCE(ta.[tag-value], 'N/A') AS [tag-value]

FROM   @AgentTableFilt ta