return sqlFnCast
	case e.Substring != nil:
		return sqlFnSubstring
	case e.Extract != nil:
		return sqlFnExtract
	case e.Trim != nil:
		return sqlFnTrim
	case e.DateAdd != nil:
		return sqlFnDateAdd
	case e.DateDiff != nil:
		return sqlFnDateDiff
	default:
		return ""
	}
}

// evalSQLFnNode assumes that the FuncExpr is not an aggregation
// function.

	Cast      *CastFunc      `parser:"| @@"`
	Substring *SubstringFunc `parser:"| @@"`
	Extract   *ExtractFunc   `parser:"| @@"`
	Trim      *TrimFunc      `parser:"| @@"`
	DateAdd   *DateAddFunc   `parser:"| @@"`
	DateDiff  *DateDiffFunc  `parser:"| @@"`

	// Used during evaluation for aggregation funcs
	aggregate *aggVal
}

// SimpleArgFunc represents functions with simple expression
// arguments.