Lesson 02 · A row - Build a SQL Database

A row

A row is an ordered list of the values you built yesterday - one per column. Today you make the row type that every table stores and every query returns.

The goal

Build a row from an ordered list of values, read a field by position, and report its length.

Start here - the target

TO DO

Scenario: Reading fields from a row

Givena row built from the values [1, "alice", null]

Whenfield at index 1 is read

Thenit is the text value "alice"

Andthe row length is 3

Andfield at index 0 is the integer value 1

Background

A row is the horizontal unit of a table: an ordered list of values, one per column, addressed by position. Column 0 is the first value, column 1 the second, and so on. Names come later - at this layer a row does not know that position 1 is called name; it only knows it holds three values and the second one is "alice".

Keeping rows positional is what lets a query pipeline stay simple: an operator can pull field 1 out of every row without carrying a schema around. The mapping from a column name to its index is a separate concern you will build next.

Make it work

// a row is just an ordered slice of Value
type Row struct { Values []Value }
func (r Row) Field(i int) Value { return r.Values[i] }
func (r Row) Len() int { return len(r.Values) }

CheckpointDONE

You can build a row and read any field by position. Commit and stop here.