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nageswaragunupudi wrote:Please read more on SELECT ... FOR UPDATE available in Oracle, MySql and some others
Please note that these are very rarely used. I strongly advise we need to unlearn DBF habits and re-orient our thinking.

We can also open recordset with pessimistic locking and if we update a record, it is locked till unlocked.

Concept of TRANSACTIONS is different.

On quite a few occasions we need to update two or more tables for recording a single transaction. We need to either alter all the tables or none of the tables but can not leave some tables altered and some unaltered.

For this purpose almost all SQL databases offer a construct like this.

BEGIN TRANSACTION
update table1
update table2, ........... etc
if there is some problem
ROLLBACK TRANSACTION
else
COMMIT TRANSACTION

The programmer can decide either to COMMIT or ROLLBACK.
Even hardware / power failures also perform an automatic ROLLBACK.

ADO provides the same facility with
oCn:BeginTrans()
oCn:CommitTrans()
oCn:RollBackTrans()

Please note TDataRow class automatically implements this transactions feature if we use more than one table and enable oRec:lUseTrans is set to .T.

Code: Select all  Expand view

function FWAdoFieldStruct( oRs, n ) // ( oRs, nFld ) where nFld is 1 based
                                    // ( oRs, oField ) or ( oRs, cFldName )
                                    // ( oField )

   local oField, nType, uval
   local cType := 'C', nLen := 10, nDec := 0, lRW := .t.  // default

   if n == nil
      oField      := oRs
      oRs         := nil
   elseif ValType( n ) == 'O'
      oField      := n
   else
      if ValType( n ) == 'N'
         n--
      endif
      TRY
         oField      := oRs:Fields( n )
      CATCH
      END
   endif
   if oField == nil
      return nil
   endif

   nType       := oField:Type

   if nType == adBoolean
      cType    := 'L'
      nLen     := 1
   elseif AScan( { adDate, adDBDate, adDBTime, adDBTimeStamp }, nType ) > 0
      cType    := 'D'
      nLen     := 8
      if oRs != nil .and. ! oRs:Eof() .and. ValType( uVal := oField:Value ) == 'T' .and. ;
            FW_TIMEPART( uVal ) >= 1.0
         cType      := 'T'
      endif
   elseif AScan( { adTinyInt, adSmallInt, adInteger, adBigInt, ;
                  adUnsignedTinyInt, adUnsignedSmallInt, adUnsignedInt, ;
                  adUnsignedBigInt }, nType ) > 0
      cType    := 'N'
      nLen     := oField:Precision + 1  // added 1 for - symbol
      if oField:Properties( "ISAUTOINCREMENT" ):Value == .t.
         cType := '+'
         lRW   := .f.
      endif
   elseif AScan( { adSingle, adDouble }, nType ) > 0
      cType    := 'N'
      nLen     := Max( 19, oField:Precision + 2 )
      nDec     := 2
   elseif nType == adCurrency
      cType    := 'N'      // 'Y'
      nLen     := 19
      nDec     := 2
   elseif AScan( { adDecimal, adNumeric, adVarNumeric }, nType ) > 0
      cType    := 'N'
      nLen     := Max( 19, oField:Precision + 2 )
      if oField:NumericScale > 0 .and. oField:NumericScale < nLen
         nDec  := oField:NumericScale
      endif
   elseif AScan( { adBSTR, adChar, adVarChar, adLongVarChar, adWChar, adVarWChar, adLongVarWChar }, nType ) > 0
      nLen     := oField:DefinedSize
      if nType != adChar .and. nType != adWChar .and. nLen > nFWAdoMemoSizeThreshold
         cType := 'M'
         nLen  := 10
      endif
   elseif AScan( { adBinary, adVarBinary, adLongVarBinary }, nType ) > 0
      nLen     := oField:DefinedSize
      if nType != adBinary .and. nLen > nFWAdoMemoSizeThreshold
         cType := 'm'
         nLen  := 10
      endif
   elseif AScan( { adChapter, adPropVariant }, nType ) > 0
      cType    := 'O'
      lRW      := .f.
   else
      lRW      := .f.
   endif
   if lAnd( oField:Attributes, 0x72100 ) .or. ! lAnd( oField:Attributes, 8 )
      lRW      := .f.
   endif

return { oField:Name, cType, nLen, nDec, nType, lRW }