缺少的月份如何填写？

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

这给了我类似的结果：

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

但是正如你所看到的，它有差距。有什么方法可以用 0 计数填充缺失的月份吗？

Answer 1

研究外连接。

运行一次：

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;