Question

我有以下SQL：

  SELECT STRFTIME('%m', date) ord,
         CASE STRFTIME('%m', date) WHEN '01' THEN 'January' 
                                   WHEN '02' THEN 'Febuary'
                                   WHEN '03' THEN 'March' 
                                   WHEN '04' THEN 'April' 
                                   WHEN '05' THEN 'May' 
                                   WHEN '06' THEN 'June' 
                                   WHEN '07' THEN 'July' 
                                   WHEN '08' THEN 'August' 
                                   WHEN '09' THEN 'September' 
                                   WHEN '10' THEN 'October' 
                                   WHEN '11' THEN 'November' 
                                   WHEN '12' THEN 'December' 
         ELSE '' END AS month,
         count(*) AS count 
    FROM events 
   WHERE type='Birth' AND 
         date <> '' 
GROUP BY month,ord
ORDER BY ord

这给我的结果类似于：

ord  month        count
01   January      1
02   Febuary      1
03   March        3
05   May          4
07   July         2
08   August       2
09   September    2
11   November     4

但是你可以看到它有差距。有没有办法用0计数来填补缺失的月份？

Answer 1

研究外部联接。

运行以下一次：

CREATE TABLE months (
  mm char(2),
  name char(10)
);
INSERT INTO months VALUES ('01', 'January');
INSERT INTO months VALUES ('02', 'Febuary');
INSERT INTO months VALUES ('03', 'March');
INSERT INTO months VALUES ('04', 'April');
INSERT INTO months VALUES ('05', 'May');
INSERT INTO months VALUES ('06', 'June');
INSERT INTO months VALUES ('07', 'July');
INSERT INTO months VALUES ('08', 'August');
INSERT INTO months VALUES ('09', 'September');
INSERT INTO months VALUES ('10', 'October');
INSERT INTO months VALUES ('11', 'November');
INSERT INTO months VALUES ('12', 'December');

然后运行：

  SELECT m.mm AS ord,
         m.name AS month,
         count(e.date) AS count
    FROM months m LEFT OUTER JOIN events e
      ON strftime('%m', e.date) = m.mm
   WHERE e.type='Birth' AND 
         e.date <> '' 
GROUP BY month,ord
ORDER BY ord;

如何填写缺失的月份？

1 个答案: