EXTRACT( ) returns a variety of elements extracted by decoding fields of date/time types. It can operate on all dialect 3 and dialect 1 date/time fields.
Here’s the syntax for EXTRACT( ):
element must be a defined element that is valid for the data type of field. Not all elements are valid for all date/time types. The data type of element varies according to the element extracted. Table enumerates the elements available for each date/ time type.
field can be a column, a variable, or an expression that evaluates to a date/time field.
Table shows the restrictions on the arguments and types when using EXTRACT( ).
EXTRACT( ) Arguments, Types, and Restrictions
Combining EXTRACT( ) with Other Functions
Following are two examples where EXTRACT( ) is used with CAST( ) to obtain date representations not available with either function on its own.
To Cast Date Plus Time Without Sub-Seconds
Although it is not possible by direct casting to return a cast date plus time string without the sub-second portion, it can be done using a complex expression involving both CAST( ) and EXTRACT( ).
To Extract a TIME String
This technique has more meaning for dialect 1 than for dialect 3. However, it can be extrapolated to any dialect 3 date or time type if you need to store time of day as a string.
The EXTRACT( ) function makes it possible to extract the individual elements of date and time types to SMALLINT values. The following trigger extracts the time elements from a dialect 1 DATE column named CAPTURE_DATE and converts them into a CHAR(13), mimicking the Firebird standard time literal 'HH:NN:SS.nnnn':
A Sample Date/Time Type Conversion Task
The CHAR(13) string stored by the trigger in the preceding example does not behave like a dialect 3 TIME type. However, by simple casting, it can be converted directly, in a later upgrade to dialect 3, to a dialect 3 TIME type.
First, we add a temporary new column to the table to store the converted time string:Next, populate the temporary column by casting the dialect 1 time string:
The next thing we need to do is temporarily alter our trigger to remove the reference to the dialect 1 time string. This is needed to prevent dependency problems when we want to change and alter the old time string:
Now, we can drop the old CAPTURE_TIME column:
Create it again, this time as a TIME type:
Move the data from the temporary column into the newly added CAPTURE_TIME:
Drop the temporary column:
Finally, fix up the trigger so that it now writes the CAPTURE_TIME value as a TIME type:
All of these steps can be written as an SQL script. For details about SQL scripting, refer to the topic “Schema Scripts” in Chapter From Drawing Board to Database.
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