Question

I have the following MYSQL select statement in a php file that was provided me by a programmer sometime ago.

I’m trying to make some edits to the file (I know enough PHP to “get myself into trouble”).

Unfortunately, I don’t understand this statement and what it means. I’ve never seen a select statement with “DOTS” in it (ie. a.weight or b.oil_type_id)

I don’t understand howto read it. Here’s the statement. Anyone know what this is supposed to mean?

$query11 = "SELECT avg(a.weight) as weight_avg
           from oil_type a, oil_data b
           where b.oil_type_id = a.oiltype_id
           and b.viscosity_id = ". $viscosity_id_this;

Answer 1

You are qualifying the columns with their table names (or rather, their aliases):

SELECT avg( a.weight ) as weight_avg 
  FROM oil_type a, oil_data b  -- b is an alias for oil_data, a is an alias for oil_type
                               -- also, there's an implicit join here
 WHERE b.oil_type_id = a.oiltype_id 
   AND b.viscosity_id =". $viscosity_id_this;

This is necessary to remove ambiguity (ie: when a column with the same name exists in both tables). If the column names are not the same, you could use this for clarity.

The whole query could be rewritten as:

    SELECT AVG(a.weight) as weight_avg 
      FROM oil_type AS a
INNER JOIN oil_data AS b 
        ON b.oil_type_id = a.oiltype_id 
     WHERE b.viscosity_id = $viscosity_id_this;

Which might be more clear (or not).