Lesson 15 · Parsing SELECT and FROM

Parsing SELECT and FROM

Now the parser produces its first statement. Today you turn "SELECT a, b FROM t" into a select node that names its columns and its table.

The goal

Parse a SELECT of a comma-separated column list and a FROM table into a select statement node, including SELECT *.

Start here - the target

TO DO

Scenario: Parsing a column list and table name

Giventhe statement "SELECT id, name FROM users"

Whenit is parsed

Thenthe result is a Select with columns ["id", "name"] and table "users"

And"SELECT * FROM users" parses with a star (all columns) marker

Background

Time for the parser’s first real grammar rule. A SELECT statement in its simplest form is the keyword SELECT, a column list, the keyword FROM, and a table name. Following the tokens: match SELECT, then read a comma-separated list of identifiers, then match FROM, then read one identifier for the table.

Give the top-level Parse one small habit now that pays off all chapter: have it peek the leading keyword and dispatch to the matching rule, returning a shared Stmt type (an empty marker interface is enough). Today only SELECT exists, so there is just one route - but CREATE and INSERT will slot in as sibling cases later without reshaping Parse. The * case is worth handling now because it is so common: SELECT * means “all columns,” which the executor treats differently from a fixed list, so record it as a flag on the node rather than a phantom column named "*". The result is a syntax tree node - a plain struct capturing what the query asked for, completely separate from how it will run.

Make it work

type Stmt interface{}   // marker: every statement node satisfies this
type SelectStmt struct { Star bool; Columns []string; From string }
// Parse(input) -> (Stmt, error): peek the leading keyword to pick a rule.
//   today only SELECT exists, so route SELECT -> parseSelect.
// parseSelect: expect(SELECT); if peek is Star -> Star=true, else a name list
//   then expect(FROM); the table is expect(Identifier)

CheckpointDONE

The parser turns a basic SELECT ... FROM into a statement node. Commit and stop here.