How to Optimize SQL Query in SQL Server

There are several ways to tune the performance of an SQL query. Here are a few tips:

1. Use appropriate indexing: Make sure that the columns used in the WHERE clause, JOIN clauses and ORDER BY clauses are indexed properly. This will help the database engine to quickly find the required data without scanning the entire table.

2. Optimize the SELECT statement: Only select the columns that are actually required for the query, instead of using the wildcard (*). Avoid using subqueries or correlated subqueries in the SELECT clause.

3. Use appropriate joins: Use the appropriate join type based on the size of the tables being joined. INNER JOINs are generally faster than OUTER JOINs. Also, use the JOIN conditions in the ON clause instead of WHERE clause.

4. Avoid using functions in WHERE clause: Functions in the WHERE clause can prevent the database engine from using indexes, which can result in slower query performance. Instead, try to rewrite the query to use the functions on the column values themselves.

5. Use UNION ALL instead of UNION: UNION ALL is faster than UNION because it doesn't remove duplicates.

6. Use LIMIT and OFFSET: If the query returns a large number of rows, use LIMIT and OFFSET to fetch only a subset of the rows. This can improve query performance significantly.

7. Monitor query performance: Use database profiling tools to identify slow queries and optimize them.

By following these tips, you can improve the performance of your SQL queries.