查询以下内容的最佳或最简单方法是什么

Question

I need help with this problem.

Question: Write an SQL query that lists all the species found in the oldest zoo in America.

I tried the following, which worked, but I would like to know if there is a simpler way to do this. I spent hours on this question.

In the first block I created a new table I called "usaT", using the INTO clause, which only hold zoos with country USA. Click data base link to see image, please. Then i query this "usaT" table in the 2nd block of code.

Thanks for the help

database photo

    SELECT * INTO usaT
    FROM zoos WHERE country='USA';

    SELECT species FROM animals, usaT
    WHERE year_founded = (SELECT MIN(year_founded) FROM usaT)
    AND animals.zoo=usaT.city;

Answer 1

• Don't use the ANSI (pre-ISO) style of SQL JOINs (ie FROM x, y WHERE x.fk = y.fk ). Always use an explicit JOIN .

• Use SELECT DISTINCT to prevent duplicate results where a zoo has more than 1 animal of the same species.

• You can get the oldest zoo by doing SELECT TOP 1... ORDER BY (in MS SQL Server)

  • In PostgreSQL (which I think you're using) you use LIMIT .

Like so:

MS SQL Server

SELECT
    DISTINCT
    animals.species
FROM
    animals
    INNER JOIN
    (
        SELECT
            TOP 1
            city
        FROM
            zoos
        WHERE
            country = 'USA'
        ORDER BY
            year_founded ASC
    ) AS oldest_zoo_in_usa ON
        animals.zoo = oldest_zoo_in_usa.city

PostgreSQL

SELECT
    DISTINCT
    animals.species
FROM
    animals
    INNER JOIN
    (
        SELECT
            city
        FROM
            zoos
        WHERE
            country = 'USA'
        ORDER BY
            year_founded ASC
        LIMIT
            1
    ) AS oldest_zoo_in_usa ON
        animals.zoo = oldest_zoo_in_usa.city

Answer 2

Query would be:

Select  s.*
from animals a join
zoos z on z.yearFounded=(Select min(year_founded) from zoos) and z.city=a.animals Join
Species s on a.species=s.species

You should assign the unique identification number to each row of the all table for simplicity and use that as primary key & foreign key relation for joins.

Answer 3

select distinct species
 from animals a
 join zoos z 
   on z.year_founded in
    (select min(year_founded)
      from zoos
     where country = 'USA')
  and a.zoo = z.city;

Hope this helps.