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Design queries iteratively. Having an understanding of relational algebra (not the symbols, but the concept; math is always about the concept) generally helps
by blattimwind 8y ago
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