What is the percentage of preprints of John Van Reenen's papers?
year = 0 means this paper is preprint; John Van Reenen is the author's name; papers refers to paper.Id; calculation = DIVIDE(SUM(paper.Id where Name = 'John Van Reenen' AND ConferenceID = 0 AND  JournalId = 0), SUM(paper.Id where Name = 'John Van Reenen'))
SELECT CAST(SUM(CASE WHEN T1.ConferenceId = 0 AND T1.JournalId = 0 THEN 1 ELSE 0 END) AS REAL) / COUNT(T1.Id) FROM Paper AS T1 INNER JOIN PaperAuthor AS T2 ON T1.Id = T2.PaperId WHERE T2.Name = 'John Van Reenen'
CREATE TABLE IF NOT EXISTS "Author"
(
    Id          INTEGER
        constraint Author_pk
            primary key,
    Name        TEXT,
    Affiliation TEXT
);
CREATE TABLE IF NOT EXISTS "Conference"
(
    Id        INTEGER
        constraint Conference_pk
            primary key,
    ShortName TEXT,
    FullName  TEXT,
    HomePage  TEXT
);
CREATE TABLE IF NOT EXISTS "Journal"
(
    Id        INTEGER
        constraint Journal_pk
            primary key,
    ShortName TEXT,
    FullName  TEXT,
    HomePage  TEXT
);
CREATE TABLE Paper
(
    Id           INTEGER
            primary key,
    Title        TEXT,
    Year         INTEGER,
    ConferenceId INTEGER,
    JournalId    INTEGER,
    Keyword      TEXT,
    foreign key (ConferenceId) references Conference(Id),
    foreign key (JournalId) references Journal(Id)
);
CREATE TABLE PaperAuthor
(
    PaperId     INTEGER,
    AuthorId    INTEGER,
    Name        TEXT,
    Affiliation TEXT,
    foreign key (PaperId) references Paper(Id),
    foreign key (AuthorId) references Author(Id)
);
