Nesting queries in SQL | 易学教程

问题

The goal of my query is to return the country name and its head of state if it's headofstate has a name starting with A, and the capital of the country has greater than 100,000 people utilizing a nested query.

Here is my query:

SELECT country.name as country, 
       (SELECT country.headofstate 
        from country 
        where country.headofstate like 'A%')      
from country, city 
where city.population > 100000;

I've tried reversing it, placing it in the where clause etc. I don't get nested queries. I'm just getting errors back, like "subquery returns more than one row" and such. If someone could help me out with how to order it, and explain why it needs to be a certain way, that'd be great.

回答1:

If it has to be "nested", this would be one way, to get your job done:

SELECT o.name AS country, o.headofstate 
FROM   country o
WHERE  o.headofstate like 'A%'
AND   (
    SELECT i.population
    FROM   city i
    WHERE  i.id = o.capital
    ) > 100000

A JOIN would be more efficient than a correlated subquery, though. Can it be, that who ever gave you that task is not up to speed himself?

回答2:

You need to join the two tables and then filter the result in where clause:

SELECT country.name as country, country.headofstate 
from country
inner join city on city.id = country.capital
where city.population > 100000
and country.headofstate like 'A%'

回答3:

The way I see it, the only place for a nested query would be in the WHERE clause, so e.g.

SELECT country.name, country.headofstate
FROM country 
WHERE country.headofstate LIKE 'A%' AND 
country.id in (SELECT country_id FROM city WHERE population > 100000)

Apart from that, I have to agree with Adrian on: why the heck should you use nested queries?

回答4:

Query below should help you achieve what you want.

select scountry, headofstate from data 
where data.scountry like 'a%'and ttlppl>=100000

来源：https://stackoverflow.com/questions/12467354/nesting-queries-in-sql

标签

sql

nested