Lesson 30 · Demo: LIMIT and a query file

Demo: LIMIT and a query file

The last operator of the chapter caps how many rows come back. Today you add LIMIT and run a whole .sql file through the engine to see the pipeline in action.

The goal

Execute a SELECT with LIMIT to return at most N rows, and run a multi-statement script through the engine.

Start here - the target

TO DO

Scenario: Limiting the number of rows

Givena users table with rows for alice (30), bob (25), carol (30)

When"SELECT name FROM users ORDER BY age DESC LIMIT 2" is executed

Thenthe result has exactly the two rows ["alice"] and ["carol"]

Anda LIMIT larger than the row count returns all rows

Background

LIMIT caps the result at the first N rows, which only means something once ordering is defined - which is why it comes last and applies after the sort. Parsing is one more trailing clause (LIMIT and a number); execution truncates the row list to at most N, guarding against a limit larger than the number of rows.

That completes the read pipeline: scan → filter → sort → limit → project, each a small operator over result sets. To close the chapter, drive a whole .sql script through the engine - several CREATE/INSERT statements to set up data, then a SELECT ... WHERE ... ORDER BY ... LIMIT that exercises the entire chain. The next chapter turns from “which rows” to “summarize the rows”: aggregates, grouping, and joins.

Make it work

// parse: optional LIMIT <number> after ORDER BY
// execute: after sorting, keep at most the first N rows
// (min of N and len(rows)); run ParseScript then Exec each statement

CheckpointDONE

The full query pipeline runs - filter, sort, limit - and a whole script executes at once. Commit and stop here.