Lesson 07 · The database - Build a SQL Database

The database

A database is a named collection of tables. Today you build the registry that creates tables by name and hands them back, so queries can refer to a table by the name in the SQL.

The goal

Create tables by name in a database, fetch them back, and reject a duplicate name.

Start here - the target

TO DO

Scenario: Creating and fetching tables by name

Givenan empty database

Whena table "users" is created and then fetched by name

Thenthe same table is returned

Andcreating "users" a second time reports an error

Andfetching an unknown table "ghosts" reports not found

Background

The database is the outermost object: a map from table name to table. When a query says FROM users, the engine asks the database for the table named "users". Creating a table registers a fresh, empty table under its name; asking for one by name hands it back so rows can be inserted or scanned.

Two rules keep it honest. Creating a table whose name already exists is an error

you should not silently clobber existing data - and fetching a name that was never created reports “not found” rather than inventing an empty table. These are the same errors a real CREATE TABLE and a query against a missing table raise, and wiring them in now means the executor gets them for free later.

Make it work

type Database struct { tables map[string]*Table }
func (db *Database) CreateTable(name string, s Schema) (*Table, error) {
  // error if name already exists, else store and return a new *Table
}
func (db *Database) Table(name string) (*Table, bool) { /* map lookup */ }

CheckpointDONE

A database holds named tables and refuses duplicates - the top-level object a query runs against. Commit and stop here.