Select TOP 100 [uniqueTradeId], [instrumentId], [MarketTradeTime]
From Transactions with (UPDLock, READPAST)
WHERE [TradeDate] is Null
Order by [MarketTradeTime]
далее в цикле для каждой записи:
update Transactions
SeT [TradeDate] = ?
Where [uniqueTradeId] = ?