Question

how do you construct mysql command to display data with m:n relationship?

i can use join to display a 1:n relationship, but when constructing m:n relationship, another table is created and when i try to do the same it, it only show one table plus the relationship table.

For example, I have :

table A:

• A_id
• A_data

table B:

• B_id
• B_data

table C:

• A_A_id
• B_B_id

I can display the table such that i have

A_id | A_data | A_A_id | B_B_id

with some data.

but the format that i want is

A_id | A_data | B_id | B_data

where A_id corresponds to B_id in table C.
I’m doing this through foreign key reference, with InnoDB engine of MySQL.

thanks

Answer 1

If you want C to be the base table for the query (ie basically take table C and add in the corresponding _data rows), you can use:

SELECT A.A_id, A.A_data, B.B_id, B.B_data
FROM C
LEFT JOIN A ON A.A_id = C.A_A_id
LEFT JOIN B ON B.B_id = C.B_B_id

This takes the table C and JOINs it to the other two tables, using C as the base table (because of the LEFT JOIN). If there are rows in C that don’t exist in A or B, you’ll just get a corresponding NULL for the _data field.

You might want to look at the various joins (INNER, LEFT, RIGHT,…) to see what behaviour you want.

For example you may want all ids that exist in table A to be shown, even if they aren’t in C (and just display a NULL for the corresponding B columns), etc.