从同一个多对多表查询

Question

I need to maintain a material table. Each material may has one or more alternative materials, so it would form a many-to-many relationship on the same table. Users can query the alternatives by a given material's part number.

I created two tables,as follows.

CREATE TABLE material (
    id int(3) NOT NULL PRIMARY KEY AUTO_INCREMENT,
    pn varchar(20) NOT NULL UNIQUE
);
CREATE TABLE mapping (
    pn_id int(3) NOT NULL,
    main_pn_id int(3) NOT NULL,
    PRIMARY KEY (pn_id, main_pn_id)
);
ALTER TABLE mapping ADD FOREIGN KEY (pn_id) REFERENCES material (id);
ALTER TABLE mapping ADD FOREIGN KEY (main_pn_id) REFERENCES material (id);

My querying inputs are always pn (part number of a material). It means the select command would look like this.

SELECT * FROM material ..... WHERE pn="XXXXX";

If I'd like to find the alternatives for a given material. I need to query its id by part number first. Then I use id to find pn_id by

SELECT pn_id FROM mapping WHERE main_pn_id=$id

At last, using pn_id to find pn from material table.

I knew it can be achieved by subquery or UNION to get alternatives for a material, but using subquery and UNION may influence the querying performance. My system may be used by hundreds of people.

I tried to use JOIN to complete a querying, but I still could not figure out how to use JOIN to query alternatives in my situation. Could anyone kindly help me on this? Thanks.

Answer 1

Why not a simple join to Material twice.

Note I used left joins here as I'm unsure if all materials would be in the mapping table.

SELECT M1.ID as MainPartID
     , M1.PN as MainPartNumber
     , M2.ID as AltPartID
     , M2.PN as AltPartNum
FROM material M1
LEFT JOIN Mapping Map
 on M1.ID = MAP.PN_ID
LEFT JOIN Material M2
 on M2.ID = MAP.Main_PN_ID
WHERE M1.PN = 'XXXXX'