Design queries iteratively.

Having an understanding of relational algebra (not the symbols, but the concept; math is always about the concept) generally helps a lot as well; it's the reason why compsci database lectures often start with relational algebra.

> We would like to find the total weight of cats grouped by age. But only return those groups with a total weight larger than 12.

The total weight of cats grouped by age.

    SELECT sum(weight), age
    FROM cats
    GROUP BY age

But only return those groups with a total weight larger than 12.

    SELECT sum(weight), age
    FROM cats
    GROUP BY age
    HAVING sum(weight) > 12

Ordered by age.

    SELECT sum(weight), age
    FROM cats
    GROUP BY age
    HAVING sum(weight) > 12
    ORDER BY age

The total weight column should be called total_weight.

    SELECT sum(weight) AS total_weight, age
    FROM cats
    GROUP BY age
    HAVING sum(weight) > 12
    ORDER BY age