Check primary keys values - sql

Question

These are tables in SQLite: especie_similar
CREATE TABLE especie_similar (
id_especie INTEGER NOT NULL,
id_similar INTEGER NOT NULL,
PRIMARY KEY (
id_especie,
id_similar
),
FOREIGN KEY (
id_especie
)
REFERENCES especie (id_especie) ON DELETE CASCADE,
FOREIGN KEY (
id_similar
)
REFERENCES especie (id_especie) ON DELETE CASCADE
);
And especie table:
CREATE TABLE especie (
id_especie INTEGER PRIMARY KEY AUTOINCREMENT
NOT NULL,
nombre_especie VARCHAR NOT NULL,
cient_especie VARCHAR NOT NULL,
desc_especie TEXT,
nidificacion TEXT NOT NULL,
dimorfismo INTEGER NOT NULL,
genero_id INTEGER NOT NULL,
endemismo_id INTEGER NOT NULL,
abundancia_id INTEGER NOT NULL,
FOREIGN KEY (
genero_id
)
REFERENCES genero (id_genero),
FOREIGN KEY (
endemismo_id
)
REFERENCES endemismo (id_endemismo),
FOREIGN KEY (
abundancia_id
)
REFERENCES abundancia (id_abundancia)
);
My situation: Table especie has multiple datas and I want to insert in especie_similar ids from table especie.
How can i code a trigger or sql statement checking if id_especie and id_similar are not equals?
How can i code to retrieve all especies distinct of target id
(ex: table especie_similar has id_especie=1 and id_similar=2
indicating that species id 1 is similar to another species id 2 but
cannot insert id_especie=2 and id_similar=1 due to previous
duplicate data)?
Thanks and sorry my english is not good.

Answer 1

For question #1 you can use a standard SQL constraint:
create table especie_similar (
...
check (id_especie <> id_similar) -- your condition here
);
For question #2 you can use a standard "Recursive CTE". See SQLite WITH Clause. Example:
with recursive node as (
select id from especie_similar
where id_especie = 123 -- first node criteria here
union all
select s.id_especie
from node n
join especie_similar s on s.id_similar = n.id
)
select * from node;