The Anatomy of a SELECT Statement

Reading Data: The Core Operation

Before joins, aggregations, or window functions, you need to understand the building blocks of every SQL query. The SELECT statement is the workhorse of SQL — it retrieves data from one or more tables.

SELECT column1, column2
FROM   table_name
WHERE  condition
ORDER BY column1 ASC
LIMIT  10;

Each clause has a specific role:

• SELECT — Which columns to return. Use * to return all columns (convenient but expensive in production — always prefer named columns).
• FROM — Which table to read from.
• WHERE — Filter rows before any grouping. Only rows matching the condition are processed further.
• ORDER BY — Sort the result set. ASC (ascending, default) or DESC (descending).
• LIMIT — Cap the number of rows returned. Critical for exploratory queries on large tables — never run SELECT * FROM orders on a 500-million-row table without a LIMIT.

The Logical Execution Order

SQL is written in one order but executed in a different order. This is one of the most common sources of confusion for new SQL writers:

1. FROM / JOIN    — Identify and assemble the source data
2. WHERE          — Filter individual rows
3. GROUP BY       — Collapse rows into groups
4. HAVING         — Filter groups
5. SELECT         — Compute the output columns
6. ORDER BY       — Sort the result
7. LIMIT / OFFSET — Truncate the result

This explains why you cannot reference a SELECT alias in a WHERE clause — the WHERE runs before SELECT computes the alias. It also explains why aggregate functions belong in HAVING, not WHERE.