Question

This is for a homework assignment. I haven’t copy-pasted the question below, I made an simpler version of it that focuses on the specific area where I’m stuck.

Let’s say I have a table of two values: a person’s name, and the place he had lunch yesterday. Assume everyone has lunch in pairs. How can I query the database to return all the pairs of people that had lunch together yesterday? Each pair must be only listed once.

I’m actually not even sure what the professor means by return them as pairs. I’ve sent him an email, but no reply yet. It seems like he wants me to write a query that returns a table with column 1 as person 1 and column 2 as person 2.

Any suggestions on how to go about this? Does it seem right to assume he wants them as separate columns?

So far, I basically have:

SELECT name, restaurant FROM lunches GROUP BY restaurant, name

which essentially just reorganizes the table so that the people who had lunch together are one after the other.

Answer 1

We have to assume there can be only one pair eating lunch in a given restaurant.
You can get a list of pairs either using self-join:

SELECT l1.name, l2.name FROM lunches l1
JOIN lunches l2
ON l1.restaurant = l2.restaurant AND l1.name < l2.name

or using GROUP BY:

SELECT GROUP_CONCAT(name) FROM lunches 
GROUP BY restaurant

The first query will return pairs in two different columns, while the second in one column, using comma as separator (default for GROUP_CONCAT, you can change it to whatever you wish).
Also note that for the first query names in pairs will come in alphabetical order as we use < instead of <> to avoid listing each pair twice.