Editing SQL (section)

== <span style="color: #FFFFFF;">Understanding</span> ==
A relational database stores data in normalized form to reduce duplication. Instead of repeating a customer's address on every order, the address lives once in a Customers table and orders reference it via a foreign key. This separation is the core idea behind relational design.

SQL queries are declarative: you describe ''what'' data you want, not ''how'' to retrieve it. The database engine (PostgreSQL, MySQL, SQLite, SQL Server, etc.) figures out the execution plan. This matters because the same logical query can be executed many different ways � and the engine picks the one it estimates to be cheapest.

How a SELECT query executes (logical order, not syntactic order):
# '''FROM / JOIN''' � identify and combine the source tables
# '''WHERE''' � filter rows before grouping
# '''GROUP BY''' � collapse rows into groups
# '''HAVING''' � filter groups
# '''SELECT''' � compute the output columns
# '''ORDER BY''' � sort the result
# '''LIMIT / OFFSET''' � slice the result set

Understanding this order explains common confusions: you cannot reference a SELECT alias in a WHERE clause (WHERE runs before SELECT), and aggregate functions like COUNT() only become valid after GROUP BY.
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