2.6 Moving Around Within a Result Set
2.6.1 Problem
You
want to iterate through a result set multiple times, or to move to
arbitrary rows within the result.
2.6.2 Solution
If your API has functions that provide these capabilities, use them.
If not, fetch the result set into a data structure so that you can
access the rows however you please.
2.6.3 Discussion
Some APIs allow you to "rewind" a result
set so you can iterate through its rows again. Some also allow you to
move to arbitrary rows within the set, which in effect gives you
random access to the rows. Our APIs offer these capabilities as
follows:
Perl DBI and Python DB-API don't allow direct
positioning within a result set.
PHP allows row positioning with the mysql_data_seek(
) function. Pass it a result set identifier
and a row number (in the range from 0 to mysql_num_rows(
)-1). Subsequent calls to row-fetching functions return
rows sequentially beginning with the given row. PHP also provides a
mysql_result( ) function that takes row and column
indexes for random access to individual values within the result set.
However, mysql_result( ) is slow and normally
should not be used.
JDBC 2 introduces the concept of a
"scrollable" result set, along with
methods for moving back and forth among rows. This is not present in
earlier versions of JDBC, although the MySQL Connector/J driver does
happen to support next( ) and previous(
) methods even for JDBC 1.12.
Whether or not a particular database-access API allows rewinding and
positioning, your programs can achieve random access into a result
set by fetching all rows from a result set and saving them into a
data structure. For example, you can use a two-dimensional array that
stores result rows and columns as elements of a matrix. Once
you've done that, you can iterate through the result
set multiple times or use its elements in random access fashion
however you please. If your API provides a call that returns an
entire result set in a single operation, it's
relatively trivial to generate a matrix. (Perl and Python can do
this.) Otherwise, you need to run a row-fetching loop and save the
rows yourself.
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