SQL ERROR

System.Data.SqlClient.SqlException (0x80131904): Column 'events.DAY_NUM' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
   at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlDataReader.TryConsumeMetaData()
   at System.Data.SqlClient.SqlDataReader.get_MetaData()
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString, Boolean isInternal, Boolean forDescribeParameterEncryption, Boolean shouldCacheForAlwaysEncrypted)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async, Int32 timeout, Task& task, Boolean asyncWrite, Boolean inRetry, SqlDataReader ds, Boolean describeParameterEncryptionRequest)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, TaskCompletionSource`1 completion, Int32 timeout, Task& task, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
   at System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.FillInternal(DataSet dataset, DataTable[] datatables, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet, String srcTable)
   at ClassCommonFuncV2.CommonFunctionsV2.dbconnect(Object sql, Object sqlname) in h:\root\home\a2zevents-001\www\site1\App_Code\CommonFunctionsV2.vb:line 121
ClientConnectionId:d0332351-61b2-4535-b616-daa4b0c443c3
Error Number:8120,State:1,Class:16

SELECT event_date as dt, EVENT_NAME as name ,path as path,Premium as premium, EVENT_INFO as description,LAST_CHECKED as lastchecked,EVENT_ID as id, SpecialOffers as sploffer,WLURL, 'Event' as tbl FROM events WHERE SpecialOffers<>'' and ( 1=0 OR WLURL='londonsalsa.co.uk' ) and ClubID=0 and (SUSPEND=0 or SUSPEND is null) and (IsDeleted=0 or IsDeleted is null) AND ( event_date >= '1784592000' OR event_end_date >= '1784592000') union SELECT null as dt, WEBSITE_NAME as name ,path as path,Premium as premium, WEBSITE_DESCRIPTION as description, LAST_CHECKED as lastchecked, WEBSITE_ID as id, SpecialOffers as sploffer, WLURL, 'Links' as tbl FROM salsa_websites WHERE SpecialOffers<>'' and ( 1=0 OR WLURL='londonsalsa.co.uk' ) union SELECT distinct day_num as dt ,EVENT_NAME as name ,path as path,Premium as premium, EVENT_INFO as description,LAST_CHECKED as lastchecked,ClubID as id, SpecialOffers as sploffer, WLURL, 'Class' as tbl FROM events WHERE ClubID>0 and SpecialOffers<>'' and ( 1=0 OR WLURL='londonsalsa.co.uk' ) and (SUSPEND=0 or SUSPEND is null) and (isDeleted=0 or isDeleted is null) AND (event_date >= '1784592000' OR event_end_date >= '1784592000') group by ClubID union SELECT null as dt, TEACHER_NAME as name ,path as path,Premium as premium, TEACHER_DESC as description,LAST_CHECKED as lastchecked,TEACHER_ID as id, SpecialOffers as sploffer, WLURL, 'Teacher' as tbl FROM teacher WHERE SpecialOffers<>'' and ( 1=0 OR WLURL='londonsalsa.co.uk' ) and (isDeleted=0 or isDeleted is null) union SELECT null as dt,VENUE_NAME as name ,path as path,Premium as premium, DESCRIPTION as description, LAST_CHECKED as lastchecked,VENUE_ID as id, SpecialOffers as sploffer,WLURL, 'Venue' as tbl FROM venue WHERE SpecialOffers<>'' and ( 1=0 OR WLURL='londonsalsa.co.uk' ) and (isDeletedV=0 or isDeletedV is null) order by Premium desc, lastchecked desc