Delphi query parameter usage when all values is also an option - sql

Question

I have a tquery (going thru BDE or BDE emulating component) that has been used to select either a single record or all records.
Traditionally this has been done as such:
select * from clients where (clientid = :clientid or :clientid = -1)
And then they would put a -1 in the field when they wanted the query to return all values. Going through this code though, I have discovered that when they have done this the query does not use proper indexing for the table and only does a natural read.
Is there a best practices method for achieving this? Perhaps a way to tell a parameter to return all values, or must the script be modified to remove the where clause entirely when all values are desired?
Edit: This is Delphi 7, by the way (And going against Firebird 1.5 sorry for leaving that out)

Answer 1

As you use deprecated BDE, that may be one more reason to migrate from BDE to 3d party solutions. AnyDAC (UniDAC, probably others too. Most are commercial libraries) has macros, which allow to dynamically change a SQL command text, depending on the macro values. So, your query may be written:
ADQuery1.SQL.Text := 'select * from clients {IF &clientid} where clientid = &clientid {FI}';
if clientid >= 0 then
// to get a single record
ADQuery1.Macros[0].AsInteger := clientid
else
// to get all records
ADQuery1.Macros[0].Clear;
ADQuery1.Open;

Answer 2

For the queries with "optional" parameters I always use ISNULL (MSSQL, or NVL Oracle), ie.
SELECT * FROM clients WHERE ISNULL(:clientid, clientid) = clientid
Setting the parameter to NULL then selects all records.
You also have to take care of NULL values in the table fields because NULL <> NULL. This you can overcome with a slight modification:
SELECT * FROM clients WHERE COALESCE(:clientid, clientid, -1) = ISNULL(clientid, -1)

Answer 3

I would use this:
SELECT * FROM CLIENTS WHERE clientid = :clientid or :clientid IS NULL

Answer 4

Using two queries is best:
if (clientid <> -1) then
begin
DBComp.SQL.Text := 'select * from clients where (clientid = :clientid)';
DBComp.ParamByName('clientid').Value := clientid;
end else
begin
DBComp.SQL.Text := 'select * from clients';
end;
DBComp.Open;
...
Alternatively:
DBComp.SQL.BeginUpdate;
try
DBComp.SQL.Clear;
DBComp.SQL.Add('select * from clients');
if (clientid <> -1) then
DBComp.SQL.Add('where (clientid = :clientid)');
finally
DBComp.SQL.EndUpdate;
end;
if (clientid <> -1) then
DBComp.ParamByName('clientid').Value := clientid;
DBComp.Open;
...

Answer 5

Remy's answer may be re-formulated as single query.
It may be better, if you gonna prepare it once and then re-open multiple times.
select * from clients where (clientid = :clientid)
and (:clientid is not null)
UNION ALL
select * from clients where (:clientid is null)
This just aggregates two distinct queries (with same results vector) together. And condition just turns one of those off.
Using would be like that:
DBComp.Prepare.
...
DBComp.Close;
DBComp.ParamByName('clientid').Value := clientid;
DBComp.Open;
...
DBComp.Close;
DBComp.ParamByName('clientid').Clear;
DBComp.Open;
However this query would rely on SQL Server optimizer capability to extract query invariant (:clientid is [not] null) and enable/disable query completely. But well, your original query depends upon that too.
Why still use obsolete FB 1.5 ? Won't FB 2.5.2 work better there ?
I think your original query is formulated poorly.
select * from clients where (:clientid = -1) or ((clientid = :clientid) and (:clientid <> -1))
would probably be easier on SQL Server optimizer. Yet i think FB could do better job there. Try to download later FB, and run your query in it, using IDEs like IBExpert or FlameRobin. Re-arranging parenthesis and changing -1 to NULL are obvious ideas to try.
Using BDE is fragile now. It is not very fast, limiting in datatypes and connectivity (no FB/IB Events for example). And would have all sorts of compatibility problems with Vista/Win7 and Win64. If FB/IB is your server of choice, consider switching to some modern component set:
(FLOSS) Universal Interbase by http://uib.sf.net (RIP all Delphi pages of http://Progdigy.com )
(FLOSS) ZeosLib DBO by http://zeos.firmos.at/
(propr) FIB+ by http://DevRace.com
(propr) IB Objects by http://IBobjects.com
(propr) AnyDAC by http://da-soft.com - sold out to Embarcadero, not-avail for D7
(propr) IB-DAC/UniDAC http://DevArt.com
Also it would be good thing to show the table and indices definition and selectivity of those indices.