Formatting: Carefully use Indentation & White spaces.
Avoid Select *
Go for the ANSI-92 JOIN Syntax.
Use Common Table Expression (CTE)
Instead of several single-row statements, use multi-row statements. A single multi-row statement is faster than several single-row statements for INSERT, UPSERT, and DELETE statements. Instead of several single-row statements, utilise multi-row statements for DML queries whenever possible.
When compared to 'INSERT... SELECT,' INTO' constructs the destination table, owns it exclusively, and is faster. Because the 'INSERT... SELECT' inserts data into an existing table, it is slower and uses more resources because there are more logical reads and transaction log consumption.
When you edit a table, the system must retain the indexes on the columns that are being changed. As a result, while having a large number of indexes can speed up select statements, it can also slow down inserts, updates, and deletes.