The SELECT operation in CSQL retrieves and structures data from a database into a result table. SELECT queries can be difficult when using aggregate functions without GROUP BY, with GROUP BY, joins, and subqueries. The diagram provided gives a brief overview of how to write a SQL query, showing common keywords, clauses, and structures like WHERE, GROUP BY, JOIN, ORDER BY, LIMIT, and more. It is a useful reference for collecting syntax for different types of SELECT queries.