SQL - database references - sql

Question

Sales table:
id | product_id | year | quantity | price
---+------------+------+----------+-------
1 | 100 | 2008 | 10 | 5000
2 | 100 | 2008 | 10 | 5000
3 | 200 | 2011 | 15 | 9000
Products table:
id | product_name | product_id
---+--------------+------------
1 | Nokia | 100
2 | Apple | 200
3 | Samsung | 200
In these two tables, I have references the Sales table and the Products table as shown here:
CREATE TABLE Sales_table
(
id SERIAL PRIMARY KEY,
product_id INTEGER NOT NULL,
year INTEGER NOT NULL,
quantity INTEGER NOT NULL,
price INTEGER NOT NULL
);
CREATE TABLE Products_table
(
id SERIAL PRIMARY KEY,
product_name VARCHAR NOT NULL,
product_id INTEGER REFERENCES Sales_table
);
These two tables are created successfully, but when I insert data into the Products_table, I get an error
ERROR: insert or update on table "products_table" violates foreign key constraint "products_table_product_id_fkey"
DETAIL: Key (product_id)=(200) is not present in table "sales_table".
By the mean of, the referenced table doesn't allow same 'product_id' field name means, then why the Products_table table has been created with the REFERENCES of Sales_table?

Answer 1

Your foreign key references are backwards. You want:
CREATE TABLE Products_table (
id SERIAL PRIMARY KEY,
product_name VARCHAR NOT NULL
);
CREATE TABLE Sales_table (
id SERIAL PRIMARY KEY,
product_id INTEGER NOT NULL,
year INTEGER NOT NULL,
quantity INTEGER NOT NULL,
price INTEGER NOT NULL,
product_id INTEGER REFERENCES product_table (id)
);
I actually advise naming the primary key after the table -- so primary key and foreign key column names match (i.e. JOINs with USING make sense). Also, in more recent versions of Postgres, generated always as identity is preferred over serial. So:
CREATE TABLE Products_table (
product_id INT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
product_name VARCHAR NOT NULL
);
CREATE TABLE Sales_table (
sale_id GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
product_id INTEGER NOT NULL,
year INTEGER NOT NULL,
quantity INTEGER NOT NULL,
price INTEGER NOT NULL,
product_id INTEGER REFERENCES products_table (product_id)
);