Left joining table to itself with WHERE

Question

I have a table called ' guests ' with these columns:

• guest_id
• wedding_id
• fname
• lname
• inviter_id

I need to select all the guests from a certain wedding_id and left join the actual inviter fname and lname instead of inviter_id (which corresponds with the guest id).

I got this far:

SELECT u.fname fname, u.lname lname, r.fname inviter_f, r.lname inviter_l 
FROM guests u LEFT JOIN guests r ON u.inviter_id = r.guest_id

The only piece of the puzzle that I'm missing is adding a:

where wedding_id=10

(to select results only for a specific wedding id).

After edit: The final query: SELECT u.fname fname, u.lname lname, r.fname inviter_f, r.lname inviter_l FROM guests u LEFT JOIN guests r ON u.inviter_id = r.guest_id where u.wedding_id=10

Now a follow up question:

How do I return empty ' ' values where there is no inviter_id(set as 0), at the current query it returns it as NULL? (inviter_f and inviter_l returns NULL)

Answer 1

试试这个查询：

SELECT u.fname fname, u.lname lname, r.fname inviter_f, r.lname inviter_l FROM guests u LEFT JOIN guests r ON u.inviter_id = r.guest_id WHERE u.wedding_id = 10

Answer 2

FROM guests u LEFT JOIN guests r ON u.inviter_id = r.guest_id and r.wedding_id = u.wedding_id where u.wedding_id = 10;