AppSync advanced filter

request.vtl

#set($expression = "#PK = :PK")
#set($expressionNames = {"#PK": "PK", "#SK": "SK"})
#set($expressionValues = {":PK": $util.dynamodb.toString("ACCOUNT#${ctx.args.accountId}")}) 
#if(!$util.isNull($ctx.args.fromDate) && $util.isNull($ctx.args.toDate))
  #set($expression = "${expression} and #SK >= :fromDate")
  $util.qr($expressionValues.put(":fromDate", $util.dynamodb.toString("TX#${ctx.args.fromDate}")))
#elseif($util.isNull($ctx.args.fromDate) && !$util.isNull($ctx.args.toDate))
  #set($expression = "${expression} and #SK <= :toDate")
  $util.qr($expressionValues.put(":toDate", $util.dynamodb.toString("TX#${ctx.args.toDate}")))
#elseif(!$util.isNull($ctx.args.fromDate) && !$util.isNull($ctx.args.toDate))
  #set($expression = "${expression} and #SK BETWEEN :fromDate AND :toDate")
  $util.qr($expressionValues.put(":fromDate", $util.dynamodb.toString("TX#${ctx.args.fromDate}")))
  $util.qr($expressionValues.put(":toDate", $util.dynamodb.toString("TX#${ctx.args.toDate}")))
#else
  #set($expression = "${expression} and begins_with(#SK, :SK)")
  $util.qr($expressionValues.put(":SK", $util.dynamodb.toString("TX#")))
#end
{
    "version" : "2017-02-28",
    "operation" : "Query",
    "limit": $util.toJson($util.defaultIfNull($ctx.args.limit, 10)),
    "scanIndexForward": false,
    "nextToken": $util.toJson($ctx.args.nextToken),
    "query" : {
      "expression": "${expression}",
      "expressionNames" : $util.toJson($expressionNames),
      "expressionValues" : $util.toJson($expressionValues)
    }
}