CREATE TABLE conference (cid INT, homepage VARCHAR, name VARCHAR)
CREATE TABLE writes (aid INT, pid INT)
CREATE TABLE cite (cited INT, citing INT)
CREATE TABLE domain_publication (did INT, pid INT)
CREATE TABLE publication_keyword (kid INT, pid INT)
CREATE TABLE organization (continent VARCHAR, homepage VARCHAR, name VARCHAR, oid INT)
CREATE TABLE publication (abstract VARCHAR, cid INT, citation_num INT, jid INT, pid INT, reference_num INT, title VARCHAR, year INT)
CREATE TABLE domain_journal (did INT, jid INT)
CREATE TABLE journal (homepage VARCHAR, jid INT, name VARCHAR)
CREATE TABLE keyword (keyword VARCHAR, kid INT)
CREATE TABLE domain_author (aid INT, did INT)
CREATE TABLE domain (did INT, name VARCHAR)
CREATE TABLE domain_conference (cid INT, did INT)
CREATE TABLE domain_keyword (did INT, kid INT)
CREATE TABLE author (aid INT, homepage VARCHAR, name VARCHAR, oid INT)

return me the references of ' Making database systems usable ' .
SELECT reference_num FROM publication WHERE title = 'Making database systems usable'
SELECT "reference_num" FROM "publication" WHERE "title" = 'Making database systems usable'