Method parseDistinctModifier

pkg/sql/parser/select.go:217–252

parseDistinctModifier parses the optional DISTINCT [ON (...)] or ALL keyword immediately after SELECT.

()

// parseDistinctModifier parses the optional DISTINCT [ON (...)] or ALL keyword
// immediately after SELECT.
func (p *Parser) parseDistinctModifier() (isDistinct bool, distinctOnColumns []ast.Expression, err error) {
	if p.isType(models.TokenTypeDistinct) {
		isDistinct = true
		p.advance() // Consume DISTINCT

		// PostgreSQL DISTINCT ON (expr, ...)
		if p.isType(models.TokenTypeOn) {
			p.advance() // Consume ON

			if !p.isType(models.TokenTypeLParen) {
				return false, nil, p.expectedError("( after DISTINCT ON")
			}
			p.advance() // Consume (

			for {
				expr, e := p.parseExpression()
				if e != nil {
					return false, nil, e
				}
				distinctOnColumns = append(distinctOnColumns, expr)
				if !p.isType(models.TokenTypeComma) {
					break
				}
				p.advance()
			}

			if !p.isType(models.TokenTypeRParen) {
				return false, nil, p.expectedError(") after DISTINCT ON expression list")
			}
			p.advance() // Consume )
		}
	} else if p.isType(models.TokenTypeAll) {
		p.advance() // ALL is the default; just consume it
	}
	return isDistinct, distinctOnColumns, nil
}

// parseTopClause parses SQL Server's TOP n [PERCENT] [WITH TIES] clause.
// Returns nil when the current dialect is not SQL Server or TOP is absent.