问题
I am new to SQL and doing the learning via datacamp. I was wondering if you can achieve the same result with 'HAVING' as with a nested 'WHERE' clause.
Related: SQL - having VS where I understand that HAVING is used with aggregate functions such as min, max, ..
How could I rewrite the following with HAVING?:
SELECT *
FROM populations
WHERE year = 2015 AND life_expectancy >(
SELECT AVG(life_expectancy)*1.15
FROM populations
);
Suppose I have 6 columns in the table 'populations': A (character), B (character), C (Character), D (number, i.e. life_expectancy as in the example), year and E (number).
I tried the following:
SELECT *
FROM populations
WHERE year = 2015
GROUP BY A, B, C, year
HAVING life_expectancy > AVG(life_expectancy)*1.15;
However, this returns an empty table. I realise it's not a reproducible example, but perhaps a general explanation as to why it might not be working would suffice :)
回答1:
HAVING is generally used to limit the results of a GROUP BY clause, in the same way that a WHERE is used to limit the results of a SELECT clause.
In addition, while a WHERE clause cannot contain aggregates directly, you can either use a HAVING clause (with the aggregates), or you could use a sub-query in the WHERE clause which contains the aggregates.
Unless you are grouping your data, or otherwise aggregating it, then I cannot see why you would want to rewrite your WHERE clause as a HAVING clause.
In your example, you are creating a set of groups, based on A, B, C and Year. Then, you are limiting those groups to the ones where the life-expectancy of the group (whatever this is meant to be?) is greater than the average life-expectancy of that same group multiplied by 1.15. It doesn't make any sense, which is what you had already established.
回答2:
HAVING works on the results, subquery is a separate query. In your example the average you calculate is for the whole table. In the HAVING clause it affects the set of data that forms the result row. So in this case it basically means you want a value to be larger than itself multiplied by 1.15 which of course doesn’t work.
HAVING is usable when you aggregate data, for example:
SELECT country, sum(population) FROM cities
GROUP BY country
HAVING sum(population) > 100000
This groups all cities by country, then returns only the rows where sum of city populations is larger than the value.
Therefore subqueries and HAVING have a very different use cases and your query can’t be turned into HAVING since you’re not trying to compare values within groups.
来源：https://stackoverflow.com/questions/49768987/what-is-the-difference-of-using-having-vs-a-subquery