Is Query of Queries faster than multiple WHERE criteria?

[EvolvedDSM]

If I need a recordset based on two table column fields (let's say firstName and lastName), is it better to run one query with a WHERE firstName = x AND lastName = y, or is it better to query of queries and find all records with a specified firstName, and then query that query to filter only records with a specified last name?  Which is the better/faster method?

<cfquery name="somequery" datasource="someds">

SELECT *

FROM sometable

WHERE firstName = 'John' AND lastName = 'Smith'

</cfquery>

or....

<cfquery name="somequery" datasource="someds">

SELECT *

FROM sometable

WHERE firstName = 'John'

</cfquery>

<cfquery name="somequery2" dbtype="query">

SELECT *

FROM somequery

WHERE lastName = 'Smith'

</cfquery>

[Dan_Bracuk]

It depends on a variety of things, but usually the fewer trips you make to the db, the better.  Also, the less data you bring back, the better.

[TLC-IT]

As a rule of thumb, make the database server do all the work... i.e. locally.  If what you are interested is "first name = x and last name = y," then those are the only records that you want to be "sent over the little wire" to you.  "The little wire" is always the bottleneck.

[EvolvedDSM]

Thanks guys.

What might be an example of when I'd like to use QoQ?  If anyone knows off-hand.

[Dan_Bracuk]

When combining data from more than one datasource.

When cfoutput query="yourquery" group="somefield" isn't going to get the job done..