在MySQL语句中使用函数而不是子选择

Question

I have the function:

function get_kwp($siteid){

$this->db->select('siteid')
         ->from('nwsite')
         ->where('siteid', $siteid);
}

I now want to include this function within another MySQL statement to save having to do a sub-select every time i need to call it.

This is the other MySQL statement that i need it to be called into:

$this->db->select("day, month, get_kwp($siteid) AS kwp")
         ->from('nwsite');

this doesn't seem to execute the function get_kwp().

Any ideas?

Thanks

Answer 1

A simple Google on CodeIgniter subqueries returned the following result: http://codeigniter.com/wiki/Subqueries

You cannot include a PHP function call in a SQL SELECT statement. You should build the subquery in your database statement. Use the link I gave to see how to actually do it.

EDIT

I just saw that the article references a library on GitHub, but the installation of this library is straightforward.

Answer 2

You can write a function in MySQL:

DELIMITER $$

CREATE FUNCTION get_kwp(Mysite_id INTEGER) RETURNS INTEGER
BEGIN
  DECLARE result INTEGER;
  SELECT siteid INTO result FROM nwsite WHERE siteid = Mysite_id LIMIT 1;
  RETURN result;
END $$

DELIMITER ;

And use that in your query:

SELECT day, month, get_kwp('$siteid') AS kwp FROM nwsite;