Question

After searching and reading a little bit I came up with the following SQL query for my application:

SELECT
  ROUND(AVG(CASE WHEN gender = 'M' THEN rating END), 1) avgAllM,
  COUNT(CASE WHEN gender = 'M' THEN rating END) countAllM,
  ROUND(AVG(CASE WHEN gender = 'F' THEN rating END), 1) avgAllF,
  COUNT(CASE WHEN gender = 'F' THEN rating END) countAllF,
  ROUND(AVG(CASE WHEN gender = 'M' AND UserAge(birth_date) <= 18 THEN rating END), 1) avgU18M,
  COUNT(CASE WHEN gender = 'M' AND UserAge(birth_date) <= 18 THEN rating END) countU18M,
  ROUND(AVG(CASE WHEN gender = 'F' AND UserAge(birth_date) <= 18 THEN rating END), 1) avgU18F,
  COUNT(CASE WHEN gender = 'F' AND UserAge(birth_date) <= 18 THEN rating END) countU18F
FROM movie_ratings mr INNER JOIN accounts a
  ON mr.aid = a.aid
WHERE mid = 5;

And I’m wondering how can I simplify this, if possible. The birth_date field is of type DATE and UserAge is a function to calculate the age from that date field.

The table structures are as follows:

[ACCOUNTS]
aid(PK), birth_date, gender

[MOVIE_RATINGS]
mid(PK), aid(PK,FK), rating

I’m looking for two things:

• General simplifications to the code above that more experienced users know about that I don’t.
• I’m doing this in PHP and for each record I’ll have an associative array with all those variables. I’m looking for a way to group them into a multidimensional array, so the PHP code is easier to read. Of course I don’t want to do this in PHP itself, it would be pointless.

For instance, something like this:

$info[0]['avgAllM']
$info[0]['countAllM']
$info[1]['avgAllF']
$info[1]['countAllF']
$info[2]['avgU18M']
$info[2]['countU18M']
$info[3]['avgU18F']
$info[3]['countU18F']

Instead of:

$info['avgAllM']
$info['countAllM']
$info['avgAllF']
$info['countAllF']
$info['avgU18M']
$info['countU18M']
$info['avgU18F']
$info['countU18F']

I don’t even know if this is possible, so I’m really wondering if it is and how it can be done.

Why I want all this? Well, the SQL query above is just a fragment of the complete SQL I need to do. I haven’t done it yet because before doing all the work, I want to know if there’s a more compact SQL query to achieve the same result. Basically I’ll add a few more lines like the ones above but with different conditions, specially on the date.

Answer 1

You could create a VIEW with the following definition

SELECT
      CASE WHEN gender = 'M' THEN rating END AS AllM,
      CASE WHEN gender = 'F' THEN rating END AS AllF,
      CASE WHEN gender = 'M' AND UserAge(birth_date) <= 18 THEN rating END AS U18M,
      CASE WHEN gender = 'F' AND UserAge(birth_date) <= 18 THEN rating END AS U18F
      FROM movie_ratings mr INNER JOIN accounts a
        ON mr.aid = a.aid
      WHERE mid = 5

Then SELECT from that

SELECT ROUND(AVG(AllM), 1) avgAllM,
       COUNT(AllM)         countAllM,
       ROUND(AVG(AllF), 1) avg,
       COUNT(AllF)         countAllF,
       ROUND(AVG(U18M), 1) avgU18M,
       COUNT(U18M)         countU18M,
       ROUND(AVG(U18F), 1) avgU18F,
       COUNT(U18F)         countU18F
FROM  yourview

Might simplify things slightly?