关于count聚合函数用法的困惑

 例题：Get the numbers and names of players who incurred more penalties than they played matches
           例题答案：select  playerno, name
                     from    players as p
                     where   (select count(*)
                              from   penalties as pen
                              where  pen.playerno = p.playerno)
                              >
                              (select count(*)
                              from   matches as m
                              where  m.playerno = p.playerno)表players结构如下：CREATE   TABLE PLAYERS
                            (PLAYERNO       INTEGER      NOT NULL,
                             NAME           CHAR(15)     NOT NULL,
                             INITIALS       CHAR(3)      NOT NULL,
                             BIRTH_DATE     DATE                 ,
                             SEX            CHAR(1)      NOT NULL,
                             JOINED         SMALLINT     NOT NULL,
                             STREET         VARCHAR(30)  NOT NULL,
                             HOUSENO        CHAR(4)              ,
                             POSTCODE       CHAR(6)              ,
                             TOWN           VARCHAR(30)  NOT NULL,
                             PHONENO        CHAR(13)             ,
                             LEAGUENO       CHAR(4)              ,
                             PRIMARY KEY    (PLAYERNO)           )表penalties结构如下：CREATE   TABLE PENALTIES
                             (PAYMENTNO      INTEGER      NOT NULL,
                              PLAYERNO       INTEGER      NOT NULL,
                              PAYMENT_DATE   DATE         NOT NULL,
                              AMOUNT         DECIMAL(7,2) NOT NULL,
                              PRIMARY KEY    (PAYMENTNO)          )表matches结构如下：CREATE   TABLE MATCHES
                            (MATCHNO        INTEGER      NOT NULL,
                             TEAMNO         INTEGER      NOT NULL,
                             PLAYERNO       INTEGER      NOT NULL,
                             WON            SMALLINT     NOT NULL,
                             LOST           SMALLINT     NOT NULL,
                             PRIMARY KEY    (MATCHNO)            )
我的疑惑是：不明白子查询的实现原理。请各位能给予指教，谢谢！