   #PHP Manual SQLite sqlite_column sqlite_create_function

   PHP Manual
   Prev  Next
   ______________________________________________________________________

                            sqlite_create_aggregate

   (no version information, might be only in CVS)
   sqlite_create_aggregate -- Register an aggregating UDF for use in SQL
   statements.

Description

   bool sqlite_create_aggregate ( resource dbhandle, string
   function_name, mixed step_func, mixed finalize_func [, int num_args])

   sqlite_create_aggregate() is similar to sqlite_create_function()
   except that it registers functions that can be used to calculate a
   result aggregated across all the rows of a query.

   The key difference between this function and sqlite_create_function()
   is that two functions are required to manage the aggregate; step_func
   is called for each row of the result set. Your PHP function should
   accumulate the result and store it into the aggregation context. Once
   all the rows have been processed, finalize_func will be called and it
   should then take the data from the aggregation context and return the
   result.

   Example 1. max_length aggregation function example
   <?php
   $data = array(
      'one',
      'two',
      'three',
      'four',
      'five',
      'six',
      'seven',
      'eight',
      'nine',
      'ten',
      );
   $dbhandle = sqlite_open(':memory:');
   sqlite_query($dbhandle, "CREATE TABLE strings(a)");
   foreach ($data as $str) {
       $str = sqlite_escape_string($str);
       sqlite_query($dbhandle, "INSERT INTO strings VALUES ('$str')");
   }
   function max_len_step(&$context, $string) {
       if (strlen($string) > $context) {
           $context = strlen($string);
       }
   }
   function max_len_finalize(&$context) {
       return $context;
   }
   sqlite_create_aggregate($dbhandle, 'max_len', 'max_len_step',
   'max_len_finalize');
   var_dump(sqlite_array_query($dbhandle, 'SELECT max_len(a) from
   strings'));
   ?>

   In this example, we are creating an aggregating function that will
   calculate the length of the longest string in one of the columns of
   the table. For each row, the max_len_step function is called and
   passed a context parameter. The context parameter is just like any
   other PHP variable and be set to hold an array or even an object
   value. In this example, we are simply using it to hold the maximum
   length we have seen so far; if the string has a length longer than the
   current maximum, we update the the context to hold this new maximum
   length.

   After all of the rows have been processed, SQLite calls the
   max_len_finalize function to determine the aggregate result. Here, we
   could perform some kind of calculation based on the data found in the
   context. In our simple example though, we have been calculating the
   result as the query progressed, so we simply need to return the
   context value.

     Note: The example above will not work correctly if the column
     contains binary data. Take a look at the manual page for
     sqlite_udf_decode_binary() for an explanation of why this is so,
     and an example of how to make it respect the binary encoding.

     Tip: It is NOT recommended for you to store a copy of the values in
     the context and then process them at the end, as you would cause
     SQLite to use a lot of memory to process the query - just think of
     how much memory you would need if a million rows were stored in
     memory, each containing a string 32 bytes in length.

     Tip: You can use sqlite_create_function() and
     sqlite_create_aggregate() to override SQLite native SQL functions.

   See also sqlite_create_function(), sqlite_udf_encode_binary() and
   sqlite_udf_decode_binary().
   ______________________________________________________________________

   Prev          Home                   Next
   sqlite_column  Up  sqlite_create_function
