REPL¶

Your Own Database Prompt¶

A REPL stands for Read-Eval-Print Loop. It's an interactive prompt where you type commands and see results immediately -- just like the Python >>> prompt, but for SQL.

pydb> SELECT * FROM cards
┌────┬────────────┬──────────┬───────┐
│ id │ name       │ type     │ power │
├────┼────────────┼──────────┼───────┤
│  1 │ Pikachu    │ Electric │    55 │
│  2 │ Charmander │ Fire     │    52 │
│  3 │ Squirtle   │ Water    │    48 │
└────┴────────────┴──────────┴───────┘
3 rows returned

pydb> SELECT name FROM cards WHERE power > 50 ORDER BY name
┌────────────┐
│ name       │
├────────────┤
│ Charmander │
│ Pikachu    │
└────────────┘
2 rows returned

pydb> .quit
Goodbye!

How to Use It¶

# Start the REPL with a database directory
pydb my_database

# Or just start with an in-memory database
pydb

Special Commands¶

Besides SQL, the REPL understands a few dot commands:

Command	What it does
`.quit` or `.exit`	Save everything and leave the REPL
`.tables`	List all tables
`.schema <table>`	Show a table's column definitions
`.save`	Save all tables to disk
`.help`	Show available commands

What We Test¶

Dot commands return the correct output (.tables, .schema, .help).
.quit, .exit and Ctrl-D save every table before the REPL stops, so nothing you typed is lost.
SQL queries are parsed, executed, and formatted as pretty tables.
Invalid SQL shows a clear parse error.
Querying a missing table shows a clear query error.
Empty results display a helpful "(empty result set)" message.

How It Works¶

The REPL is just a loop:

Read -- show the pydb> prompt and wait for input.
Eval -- parse the SQL and execute it against the database.
Print -- format the results as a nice table.
Loop -- go back to step 1.

That's it! The REPL ties together everything we've built: the SQL parser, the query executor, the database, and the storage engine.

Next Up¶

The REPL is the front door to your database. Now explore more advanced features like Transactions for the "no take-backs" rule.