内部联接两个表，都具有没有主键的外键？

Question

TABLE_1(G_id(fk),A_id(fk),description)
TABLE_2(U_id(fk),G_id(fk),Permision)

The G-id is a primary key of Table 3 that is foreign to table 1 and table 2.

TABLE_3(G_id(pk),name,des)

Now I am applying an inner join on table 1 and table 2 on the basis of G_id without using table 3 but it showing no record found. Why?

SELECT *
FROM TABLE_1 INNER JOIN TABLE_2
ON TABLE_1.G_id=TABLE_2.G_id

Answer 1

There is no rule like join should have primary key or foreign key.

A table without any key, index can be part of join. Keys are to ensure your integrity.

But defining keys are good for your data integrity and performance as some cases, SQL Server optimizer will be smart enough to understand your relation of the tables through the keys and perform better.

SELECT TABLE_1.* ,TABLE_2

FROM TABLE_1 INNER JOIN TABLE_2

ON TABLE_1.G_id=TABLE_2.G_id

The kind of table you're describing is sometimes called a join table . You would create a primary key across the columns you're joining, both for uniqueness and because it's more likely to be indexed for better performance.