how does a SQL query work?

Question

How does a SQL query work? How does it get compiled? Is the from clause compiled first to see if the table exists? How does it actually retrieve data from the datab

Answer 1

The order of SQL statement clause execution-

FROM -> WHERE -> GROUP BY -> HAVING -> SELECT -> ORDER BY

My answer is specific to Oracle database, which provides tutorials pertaining to your queries. Well, when SQL database engine processes any SQL query/statement, It first starts parsing and within parsing it performs three checks Syntax, Semantic and Shared Pool. To know how do these checks work? Follow the link below.

Once query parsing is done, it triggers the Execution plan. But hey Database Engine! you are smart enough. You do check if this SQL query has already been parsed (Soft Parse), if so then you directly jump on execution plan or else you deep dive and optimize the query (Hard Parse). While performing hard parse, you also use a software called Row Source Generation which provides Iterative Execution Plan received from optimizer. Enough! see the SQL query processing stages below.

Note - Before execution plan, it also performs Bind operations for variable's values and once the query is executed It performs Fetch to obtain the records and finally store into result set. So in short, the order is-

PASRE -> BIND -> EXECUTE -> FETCH

And for in depth details, this tutorial is waiting for you. This may be helpful to someone.