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

Well...

• First you have a syntax check, followed by the generation of an expression tree - at this stage you can also test whether elements exist and "line up" (i.e. fields do exist WITHIN the table). This is the first step - any error here any you just tell the submitter to get real.
• Then yo have.... analysis. A SQL query is different from a program in that it does not say HOW to do something, just WHAT THE RESULT IS. Set based logic. So you get a query analyzer in (depending on product bad to good - oracle long time hat crappy ones, DB2 the most sensitive ones even measuring disc speed) to decide how best to approach this result. This is a really complicated beast - it may try dozens or hundreds of approaches to find one he believes to be fastest (cost based, basically some statistics).
• Then that gets executed.

The query analyzer, by the way, is where you see huge differences. Not sure about MySQL - SQL Server (Microsoft) shines in that it does not have the best one (but one of the good ones), but that it really has nice visual tools to SHOW the query plan, compare the estimates the the analyzer to the real needs (if they differ too much table statistics may be off so the analyzer THINKS a large table is small). They present that nicely visually.

DB2 had a great optimizer for some time, measuring - i already said - disc speed to put it into it's estimates. Oracle went "left to right" (no real analysis) for a long time, and took user provided query hints (crap approach). I think MySQL was VERY primitive too in the start - not sure where it is now.

Table format in database etc. - that is really something you should not care for. This is documented (clearly, especially for an open source database), but why should you care? I have done SQL work for nearly 15 years or so and never had that need. And that includes doing quite high end work in some areas. Unless you try building a database file repair tool.... it makes no sense to bother.

Answer 2

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.