Postgres 11

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CREATE TABLE customer_order(
 order_id serial PRIMARY KEY NOT NULL,
 customer_name VARCHAR(100) NOT NULL
);

CREATE TABLE order_detail(
    id serial PRIMARY KEY,
    order_id INTEGER,
    item_sku VARCHAR(50),
    item_quantity INTEGER,
    UNIQUE (order_id, item_sku),
    FOREIGN KEY(order_id) REFERENCES customer_order(order_id)
);

INSERT INTO customer_order (customer_name) VALUES
 ('John')
,('Mary')
,('Bill')
,('Rich'); -- added

INSERT INTO order_detail (order_id, item_sku, item_quantity) VALUES
 (1, 'APPLE' , 2)
,(1, 'ORANGE', 3)
,(2, 'ORANGE', 5)
,(3, 'APPLE' , 2)
,(3, 'ORANGE', 3)
,(4, 'APPLE' , 2)
,(4, 'ORANGE', 3)
,(4, 'BANANA', 1);  -- additional item

CREATE TABLE pack_master(
 id serial PRIMARY KEY NOT NULL,
 name VARCHAR(100) NOT NULL
);

CREATE TABLE pack_child(
    id serial PRIMARY KEY,
    pack_id INTEGER,
    item_sku VARCHAR(50),
    item_quantity INTEGER,
    UNIQUE (pack_id, item_sku),
    FOREIGN KEY(pack_id) REFERENCES pack_master(id)
);

INSERT INTO pack_master (name) VALUES
 ('Pack A')
,('Pack B');

INSERT INTO pack_child (pack_id, item_sku, item_quantity) VALUES
 (1, 'APPLE' , 2)
,(1, 'ORANGE', 3)
,(2, 'GRAPES', 5);

CREATE TABLE

CREATE TABLE

INSERT 0 4

INSERT 0 8

CREATE TABLE

CREATE TABLE

INSERT 0 2

INSERT 0 3

pack_id	order_id
1	1
1	3

SELECT 2