Question

I’ve been racking my brain trying to figure out how to accomplish a link through foreign keys to help make life easier. I have two tables set up and want to make a 3rd linking to them both adding a couple columns to the 3rd one. here’s the syntax I came up with…

SQL Fiddle (with sample data)

Shirts

CREATE TABLE shirts(
    shirt_id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
    shirt_name VARCHAR(100) NOT NULL,
    shirt_type VARCHAR(10) NOT NULL,
    shirt_size VARCHAR(20) NOT NULL,
    qp_price_id VARCHAR(20) NOT NULL,
    o_price_id VARCHAR(20) NOT NULL
)ENGINE=INNODB;

Tthere will be more with the V-Necks, Raglan Sleeve, Cap Sleeve etc. shirts….

Price List

CREATE TABLE price_list(
    price_id INT UNSIGNED AUTO_INCREMENT NOT NULL PRIMARY KEY,
    price_cat VARCHAR(20) NOT NULL,
    price NUMERIC(6,2) NOT NULL
)ENGINE=INNODB

what I would like to do is create a 3rd table that selects the shirt_name, shirt_type, and shirt_size columns from from the shirts table then create two columns, one being qp_price and the other o_price and make those foreign keys to the price list table to grab the price for that shirt… and I figured maybe using a WHERE clause somehow where the price that shows is controlled by

WHERE shirts.qp_price_id=price_list.price_cat AND shirts.o_price_id=price_list.price_cat

I want to do it this way so I can make price changes as easily as possible to multiple items. This chunk is just for each variety of crewneck shirts I need to represent… there’s more… all the crewneck shirts cost the same price from New Born- 1X adult, so I have those attributed to “crn-qp-a” and “crn-o-a” so if I can just change the price value of “crn-qp-a” ONE TIME and have it automatically update to 20 instances so I don’t have to do it 20 times…. does this make sense?…. please help… lol

Answer 1

You want to, first, remove price information from shirts table and keep it price_list, and then create the third table, and in it you can can link shirts to prices by shirt_id and price_id

create table shirt_price (
  shirt_id int UNSIGNED not null,
  qp_price_id int UNSIGNED not null,
  o_price_id int UNSIGNED not null,
  primary key (shirt_id, price_id),
  CONSTRAINT shirt_price_ibfk_1 FOREIGN KEY (shirt_id) REFERENCES shirts (shirt_id),
  CONSTRAINT shirt_price_ibfk_2 FOREIGN KEY (qp_price_id) REFERENCES price_list (price_id),
  CONSTRAINT shirt_price_ibfk_3 FOREIGN KEY (o_price_id) REFERENCES price_list (price_id)   
)ENGINE=INNODB;

So for example, if your shirts table has the following shirts:

SHIRT_ID    SHIRT_NAME      SHIRT_TYPE  SHIRT_SIZE  
1           Crewneck Tee    Men         S
2           Crewneck Tee    Men         M
3           Crewneck Tee    Men         L
4           Crewneck Tee    Men         1X
5           Crewneck Tee    Men         2X

and your price_list table has the following prices:

PRICE_ID    PRICE_CAT   PRICE
1           crn_qp_a    27.2
2           crn_o_a     25.31
3           crn_qp_b    28.1

Then each shirt will link to 2 prices, one for the qp price, and the other for the o price.

SHIRT_ID    QP_PRICE_ID   O_PRICE_ID
1           1             2
2           1             2
3           1             2
4           3             2          
5           1             2           

To query the price of all men shirts, you do the following:

select s.shirt_name, s.shirt_type, s.shirt_size, p_qp.price, p_o.price
from shirts s,
   price_list p_qp,
   price_list p_o,
   shirt_price rel
where s.shirt_type = 'Men'
  and rel.shirt_id = s.shirt_id
  and rel.qp_price_id = p_qp.price_id
  and rel.o_price_id = p_o.price_id;

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