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The web version only has simple instructions since chapter 04, while the full book has detailed explanations and background info.

0505: SELECT Expression

Modify Structs

In this step, we’ll allow expressions in SELECT and UPDATE:

select a * 4 - b, d + c from t where d = 123;
update t set a = a - b, b = a, c = d + c where d = 123;

In StmtSelect, cols changes from string to interface{}. StmtUpdate is updated in a similar way, adding ExprAssign:

type StmtSelect struct {
    table string
    // cols  []string
    cols  []interface{} // ExprUnOp | ExprBinOp | string | *Cell
    keys  []NamedCell
}
type StmtUpdate struct {
    table string
    keys  []NamedCell
    // value []ExprEqual
    value []ExprAssign
}
type ExprAssign struct {
    column string
    expr   interface{} // ExprUnOp | ExprBinOp | string | *Cell
}

Modify Parser

func (p *Parser) parseAssign(out *ExprAssign) (err error) {
    var ok bool
    out.column, ok = p.tryName()
    if !ok {
        return errors.New("expect column")
    }
    if !p.tryPunctuation("=") {
        return errors.New("expect =")
    }
    out.expr, err = p.parseExpr()
    return err
}

Modify Execution

Update select and update to call evalExpr() to produce results:

func (db *DB) execSelect(stmt *StmtSelect) ([]Row, error)
func (db *DB) execUpdate(stmt *StmtUpdate) (count int, err error)

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