MySQL-SQL and Relational Algebra queries

Question

This section asks you to write some queries in Relational Algebra and SQL.Your SQL queries will be graded by running them in MySQL on test cases; queries that return the wrong answer will receive 0 points, i.e. there is no partial credit. In particular, please make sure your query returns the right column(s).

Consider the schema given by the following two SQLCREATEstatements.

CREATE TABLE SuppInfo (suppid INTEGER NOT NULL,
prodid INTEGER NOT NULL,
PRIMARY KEY (suppid, prodid));

CREATE TABLE Purchases (purchaseid INTEGER PRIMARY KEY,
custid INTEGER,
prodid INTEGER,
purchasemethod INTEGER);

Answer 1

relational algebra这么高深的玩意就不做了.

mysql:

题一:
select prodid, group_concat(suppid) from SuppInfo group by prodid;

题二:
select custid from Purchases group by custid having count(distinct purchasemethod)>1;
select custid from Purchases P1 where exists (select 1 from Purchases P2 where P1.purchasemethod <> P2.purchasemethod );