Mysql 统计列数

Question

我有一个包含四列的图片库表，例如：

foid | uid | pic1  | pic2  | pic3  | date     |
-----------------------------------------------
104  |  5  | 1.jpg | 2.jpg | 3.jpg | 2010-01-01
105  | 14  | 8.jpg |       |       | 2009-04-08
106  | 48  | x.jpg | y.jpg |       | 2010-08-09

用户画廊的Mysql查询如下：

SELECT * FROM foto WHERE uid = $id order by foid DESC

我想做的是计算每个列出的画廊中的图像数量（PIC1、PIC2、PIC3）。

最好的方法是什么？

Answer 1

我假设每个foid 代表一个画廊：

SELECT foid, uid, date,
       (CASE WHEN pic1 IS NULL THEN 0 ELSE 1 END +
        CASE WHEN pic2 IS NULL THEN 0 ELSE 1 END +
        CASE WHEN pic3 IS NULL THEN 0 ELSE 1 END
       ) AS pic_count
  FROM foto
 WHERE uid = $id
 ORDER BY foid DESC

顺便说一下，这不是一种特别合理的架构架构方式。您真的应该将图片拆分为单独的表格：

foto:
foid | uid | date
-----------------------
104  |  5  | 2010-01-01
105  | 14  | 2009-04-08
106  | 48  | 2010-08-09

pic:
foid | pic
------------
104  | 1.jpg
104  | 2.jpg
104  | 3.jpg
105  | 8.jpg
106  | x.jpg
106  | y.jpg

现在，画廊可以有超过三张图片，查询更简单：

SELECT foid, uid, date, COUNT(*)
  FROM foto
  JOIN pic USING (foid)
 WHERE uid = $id
 ORDER BY foid DESC
 GROUP BY foid, uid, date

编辑：您可以这样拆分现有数据库（只是猜测列类型之类的东西）：

CREATE TABLE picture (foid INT, pic VARCHAR(255));

INSERT INTO picture (foid, pic)
        SELECT foid, pic1 as pic FROM foto WHERE pic IS NOT NULL
        UNION
        SELECT foid, pic2 as pic FROM foto WHERE pic IS NOT NULL
        UNION
        SELECT foid, pic3 as pic FROM foto WHERE pic IS NOT NULL
    ;

ALTER TABLE foto
    DROP COLUMN pic1,
    DROP COLUMN pic2,
    DROP COLUMN pic3
    ;

显然，删除列时应格外小心，并在开始之前进行备份！