Lesson 19 · The WHERE clause - Build a SQL Database

The WHERE clause

Now you attach an expression to a query. Today you extend the SELECT parser to read an optional WHERE clause and hang its condition on the statement node.

The goal

Parse an optional WHERE clause on a SELECT, storing its condition expression, or leaving it empty when absent.

Start here - the target

TO DO

Scenario: Parsing a SELECT with a WHERE clause

Giventhe statement "SELECT * FROM users WHERE age > 18"

Whenit is parsed

Thenthe Select has a Where expression Compare(ColRef(age), ">", IntLit(18))

And"SELECT * FROM users" parses with no Where (empty)

Background

The SELECT node from lesson 15 knows its columns and table; now it gains a condition. After the FROM table, the parser peeks for the WHERE keyword - if present, it consumes it and parses a full boolean expression using the precedence-aware rules you just built, storing the resulting tree on the statement. If WHERE is absent, the condition is simply empty and the query matches every row.

This is the moment the parser’s two halves meet: statement parsing (lesson 15) and expression parsing (lessons 16-18) compose into one tree. An optional clause like this is just a peek and a conditional parse - the same pattern you will reuse for ORDER BY, LIMIT, and every other trailing clause the query language grows.

Make it work

// after parsing FROM <table>:
//   if peek is Keyword(WHERE): consume it, parse a full expression
//   store it on SelectStmt.Where (nil when the clause is absent)

CheckpointDONE

A SELECT can now carry a WHERE condition parsed as a full expression. Commit and stop here.