SQL Fundamentals

SELECT, WHERE, and JOIN — retrieving and combining rows from relational tables.

What it is

SQL queries retrieve rows matching conditions and combine data across tables via joins.

Core clauses

SELECT / FROM / WHERE — pick columns, pick a table, filter rows before any grouping happens.
JOIN types — INNER JOIN keeps only matching rows in both tables; LEFT JOIN keeps every row from the left table, with NULLs where there's no match on the right; RIGHT/FULL JOIN follow the same idea from the other side or both.
ORDER BY / LIMIT — sort the result set and cap how many rows come back.

Execution order (not written order)

SQL is written SELECT ... FROM ... WHERE ... GROUP BY ... ORDER BY, but conceptually executes FROM → WHERE → GROUP BY → SELECT → ORDER BY — which is why you can't reference a SELECT alias in the same query's WHERE clause.

Why it's the root of the SQL topic

Aggregation, window functions, indexing, and schema design all assume fluency with joins and filtering first — they're refinements on top of "retrieve the right rows."

Leads to: Aggregations & GROUP BY, Schema Design & Normalization