带Union子句的简单Oracle SQL查询

Question

is it possible to write SQL query to retrieve the names of person with animals named 'roxy'? If so please tell me how to write the query. I don't know Oracle so I'm confused of this foreign key with reference.

Also, shouldn't we add an index for person_id otherwise it will create a speed issue later on when doing union? How to write the query to add that index? or does the reference create the index as well?

  create table person (
    person_id   integer,
    name        varchar(50)
  );

  create table animal(
    owner_id    integer,
    name        varchar(50),
    foreign key (owner_id) references person (person_id)
  );

Answer 1

Oracle uses just the same syntax for joins than SQL Server.

select * from person p inner join animal a on p.person_id = a.owner_id

Also, you should set primary keys on your tables, and put an index on them.

Left Join are used when you wish to obtain ALL elements from the "left" table, even if they dont have a match on the joining table.

On oracle you would create a primary key as follows:

CREATE UNIQUE INDEX XPKperson ON person(person_id)

Answer 2

You can use the below query,

SELECT p.name
FROM   person p,
       animal a
WHERE  p.person_id = a.owner_id
AND    a.name = 'roxy';

You should create a primary keys on tables as it uniquely defines a record.

Syntax to create an index,

CREATE INDEX index_name ON TABLE_NAME(column_name);

Refer this link to understand more about creating an index.