How can I choose which column do I refer to? - sql

Question

I have 2 tables with some duplicate columns. I need to join them without picking which columns I want to select:
CREATE TABLE IF NOT EXISTS animals (
id int(6) unsigned NOT NULL,
cond varchar(200) NOT NULL,
animal varchar(200) NOT NULL,
PRIMARY KEY (id)
) DEFAULT CHARSET=utf8;
INSERT INTO animals (id, cond, animal) VALUES
('1', 'fat', 'cat'),
('2', 'slim', 'cat'),
('3', 'fat', 'dog'),
('4', 'slim', 'dog'),
('5', 'normal', 'dog');
CREATE TABLE IF NOT EXISTS names (
id int(6) unsigned NOT NULL,
name varchar(200) NOT NULL,
animal varchar(200) NOT NULL,
PRIMARY KEY (id)
) DEFAULT CHARSET=utf8;
INSERT INTO names (id, name, animal) VALUES
('1', 'LuLu', 'cat'),
('2', 'DoDo', 'cat'),
('3', 'Jack', 'dog'),
('4', 'Shorty', 'dog'),
('5', 'Stinky', 'dog');
SELECT *
FROM animals AS a
JOIN names as n
ON a.id = n.id;
Result:
| id | cond | animal | id | name | animal |
| --- | ------ | ------ | --- | ------ | ------ |
| 1 | fat | cat | 1 | LuLu | cat |
| 2 | slim | cat | 2 | DoDo | cat |
| 3 | fat | dog | 3 | Jack | dog |
| 4 | slim | dog | 4 | Shorty | dog |
| 5 | normal | dog | 5 | Stinky | dog |
But when I try to make another request from the resulting table like:
SELECT name
FROM
(
SELECT *
FROM animals AS a
JOIN names as n
ON a.id = n.id
) as res_tbl
WHERE name = 'LuLu';
I get:
Query Error: Error: ER_DUP_FIELDNAME: Duplicate column name 'id'
Is there any way of avoiding it except removing duplicate columns from the 1st request?
P.S. in fact I am using PostgreSQL, I create my schema as MySQL because I am more used to it

Answer 1

You have columns with the same name in both tables, which causes ambiguity.
If you just want the name column in the outer query, then select that column only in the subquery:
select name
from (
select n.name
from animals a
inner join names n using (id)
) t
where ...
If you want more columns, then you would typically alias the homonym columns to remove the ambiguity - as for the joining column (here, id), the using() syntax is sufficient. So, for example:
select ...
from (
select id, a.cond, a.animal as animal1, n.name, n.animal as animal2
from animals a
inner join names n using (id)
) t
where ...
You may also select the records themselves, instead of the columns from them, which you can then access in an outer query using the usual record.column syntax;
SELECT a.cond animal_cond,
n.name animal_name
FROM (
SELECT a, n
FROM animals AS a
JOIN names as n
ON a.id = n.id
) t