How to Write Optimal SQL Queries

Question

I\'ve searched around stackoverflow but everybody asks to optimize queries they\'ve already done.

I want to know, the basic stuff on what to do, what to avoid when creat

Answer 1

My simple rules to write a query:

1. Write FROM clause from the most smallest table. This helps to find data more efficiently as we make searching in smaller amount of data.

2. At first you should write INNER JOIN, then LEFT OUTER JOIN. This helps to decrease quantity of rows where SQL Engine will search your data.

   For example:

   SELECT 
       pe.Name,
       de.Name,
       bu.Name
   FROM dbo.Persons pe
   INNER JOIN dbo.Departments de ON pe.ID = de.id_Person -- at first INNER JOIN
   LEFT JOIN dbo.Bureau bu ON bu.ID = de.id_Bureau -- then LEFT OUTER JOIN
   

3. Use aliases and schema name to avoid schema scanning by SQL Server. As using schema name helps to cashe your query plan for ad-hoc queries that can be reusable by other users, not only for your queries.

4. Avoid using SELECT * ...