Accessing list data in multiple site collections

SharePoint Search Query APIs is a way to go for accessing list data across site collections.

Examples

Assume we need to retrieve Task items, then the following query could be used:

contentclass:STS_ListItem AND ContentType:Task

to return all the task items.

JavaScript REST example

The following example demonstrates how to utilize Search Query API using REST endpoint:

function searchTaskItems(webUrl,success, failure) {
    var url = webUrl + "/_api/search/query?querytext='contentclass:STS_ListItem AND ContentType:Task'";
    $.ajax({
        url: url,
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" },
        success: function (data) {
            success(data.d.query);
        },
        error: function (data) {
            failure(data);
        }
    });
}


//print tasks
searchTaskItems(_spPageContextInfo.webAbsoluteUrl,
  function(query){
      var resultsCount = query.PrimaryQueryResult.RelevantResults.RowCount;
      for(var i = 0; i < resultsCount;i++) {
          var row = query.PrimaryQueryResult.RelevantResults.Table.Rows.results[i];
          var taskName = row.Cells.results[3].Value;
          console.log(taskName);
      }   
  },
  function(error){
    console.log(JSON.stringify(error));
  }
);

JSOM Example

The same example but using JSOM:

SP.SOD.executeFunc('SP.search.js', 'Microsoft.SharePoint.Client.Search.Query.KeywordQuery', function() {

  var context = SP.ClientContext.get_current();
  var site = context.get_site();
  var keywordQuery = new Microsoft.SharePoint.Client.Search.Query.KeywordQuery(context); 
  keywordQuery.set_queryText("contentclass:STS_ListItem AND ContentType:Task"); 
  var searchExecutor = new Microsoft.SharePoint.Client.Search.Query.SearchExecutor(context);  
  var results = searchExecutor.executeQuery(keywordQuery); 
  context.executeQueryAsync(
    function(){
        for(var idx in results.get_value().ResultTables[0].ResultRows) {
            var taskTitle = results.get_value().ResultTables[0].ResultRows[idx].Title;
            console.log(taskTitle);    
        }
    }, 
    function(sender,args){
        //error handling goes here...
    });

});    

References

• Using the SharePoint 2013 search Query APIs

SharePoint Search is the best way to do this in 2013.

Here are some walkthroughs:
http://www.sharepointnutsandbolts.com/2012/10/using-content-search-web-part-and.html

http://channel9.msdn.com/Events/SharePoint-Conference/2014/SPC402

http://www.dotnetmafia.com/blogs/dotnettipoftheday/archive/2013/04/09/how-to-query-sharepoint-2013-using-rest-and-javascript.aspx