National Instruments BridgeVIEW 用户手册

Appendix B

Citadel and Open Database Connectivity

B-4

How Do You Access Citadel Data?

The ODBC driver presents Citadel data to other applications as a Threads
table. The table contains a field or column for each data member logged to
the Citadel database.

Threads Table

The Threads table contains three fields you can use to specify query criteria
and to time-stamp retrieved data:

Interval

LocalTime

, and

UTCTime

Interval allows you to specify the query value sample rate. Interval can
range from 10ms to several years. By default, Interval is 1 (one day).

Remember, Citadel only logs a value when the value changes (it is
event-driven). But using Interval, you can query Citadel for values evenly
spaced over a period of time.

LocalTime and UTCTime indicate the time-stamps of when values are
logged. Citadel actually stores the time in UTCTime format and derives
LocalTime from the stored time. When you do not specify a time, Citadel
assumes midnight of the current day.

The following where clause from a query takes advantage of Interval

and

LocalTime to select data over a specified time at one minute intervals.
Notice that time and date formats are the same as those used in
BridgeVIEW.

SELECT * FROM Threads

WHERE LocalTime > "12/1 10:00"

AND LocalTime < "12/2 13:00"

AND Interval = "1:00"

Data Transforms

Your queries can include special commands that perform data transforms,
making it easy to manipulate and analyze historical data. The following
table lists data transform commands.