How would you optimize a slow-performing SQL Server query?

To optimize a slow-performing SQL Server query, you can follow these best practices:

1. Ensure proper indexing: Use appropriate indexes on columns involved in the WHERE, JOIN, and ORDER BY clauses of the query. This can significantly improve query performance.

2. Use SQL Profiler: SQL Profiler can help identify inefficient queries by capturing live query execution data. Analyzing this data can reveal areas for optimization.

3. Update Statistics: Keeping statistics up-to-date helps the query optimizer make better decisions when generating query execution plans.

4. Avoid using SELECT *: Instead of selecting all columns in a table, specify only the required columns. This can reduce the amount of data processed and improve query performance.

5. Use SET NOCOUNT ON: Turning off the "X row(s) affected" message can reduce network traffic and improve query performance.

6. Consider query rewriting: Rewriting complex queries to simpler forms can sometimes improve query performance by allowing the optimizer to generate more efficient execution plans.

7. Implement query caching: Use stored procedures or query caching mechanisms to store and reuse frequently executed queries, reducing the overhead of query compilation.

By incorporating these techniques, you can effectively optimize slow-performing SQL Server queries and enhance overall database performance.