How to select in 3 tables?

Question

I need to do SELECT in two tables with relationship n for n, soon, I will have to make use of the intermediate table as well, but I don’t know how to do it. I am using PHP and Sqlserver database.

Answer 1

You need to list all 3 tables using the JOIN command See a simple example.

SELECT * FROM tabelaA a
INNER JOIN tabelaRelacao r on a.id_tabelaA = r.id_tabelaA
INNER JOIN tabelaB b on a.id_tabelaB = r.id_tabelaB

Follows an image that graphically represents the Use of joins.

Answer 2

You need to make use of JOIN in your consultation.

SELECT
    T.ID_Tabela1,
    T.Descricao,
    T3.Descricao
FROM Tabela1 T
INNER JOIN
    Tabela2 T2 ON T2.ID_Tabela2 = T.ID_Tabela2
INNER JOIN
    Tabela3 T3 ON T3.ID_Tabela3 = T2.ID_Tabela3

Answer 3

Use joins:

Inner Join: Registration required under A and B Left Join: Registration required on A, B may be null Right Join: Registration required on B, A may be null

Tutorial: http://www.devmedia.com.br/clausulas-inner-join-left-join-e-right-join-associacao-de-tabelas-sql-server-2008-parte-2/18930

SELECT
    t1.nome, t1.id01, t2.id01, t2.id02, t3.curso, t3.id02
FROM
    TABELA01 t1
    INNER JOIN TABELA02 t2
    ON t1.id01 = t2.id01
    INNER JOIN TABELA 03 t3
    ON t2.id02 = t3.id02

Answer 4

Let’s take an example:

You have an Article object that has more than one category, and a category may have more than one article.

In this example I want to pick up 10 articles with all categories.

SELECT * FROM artigo

LEFT JOIN artigo_categorias ON (artigo.id = artigo_categorias.artigo_id)
LEFT JOIN categorias ON (artigo_categorias.categoria_id = categoria_id)

LIMIT 10

In this case you could just make the call you make in PHP by calling this SQL.