Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
Date: Mon, 10 Mar 2003 10:05:13 +0200
Lauri Pietarinen wrote:
> 1) SQL operates in two modes: "bag"-mode and "set"-mode
> 2) The user uses the "set" mode when he either
> - includes a candidate key in the result (implicit "set" mode)
> - or uses the keyword DISTINCT (explicit "set" mode)
> 3) The user uses the "bag" mode when he does not
> include a candidate key in the result and does
> not specify DISTINCT (implicit "bag" mode)
I forgot these points:
3a) Certain optimisations are available ONLY if the user "chooses" the "set"-mode AND the DBMS happens to support this optimising
re: select p# from v_s_sp_p <--> select distinct p# from v_s_sp_p
3b) As a consequence the user has to be very knowledgeble on the properties of the DBMS and the distinction between "set" mode and "bag" mode.
He would also have to, in the case of v_s_sp_p understand the "inner workings" of the view, hence lowering the level of absraction.
Lauri Pietarinen Received on Mon Mar 10 2003 - 09:05:13 CET