MySQL JOIN BY 到多列

Question

我無法弄清楚以下查詢。

我有3張桌子。

旗幟

id     flag 
---------------
1      Belgium
2      France 

顏色

id    color
---------------
1     Red
2     Yellow
3     Blue
4     Black
5     White

Flag_Colors

id    flag_id  color_id
------------------------
1        1        1
2        1        2
3        1        4
4        2        1
5        2        5
6        2        3

我想得到以下結果：

Flag     Color_1    Color_2    Color_3
----------------------------------------
Belgium  Red        Yellow     Black
France   Red        White      Blue

到目前為止，我有以下幾點：

SELECT 
    f.flag, c.color as color_1
FROM 
    flags f

LEFT JOIN
    flag_colors fc
    ON fc.flag_id = f.id

LEFT JOIN
    colors c
    ON c.id = fc.color_id

GROUP BY f.id

提前致謝。

Answer 1

你可以用一個函數來做到這一點。 如果您不能使用客戶端代碼；)

這是一個解決方案（處理“無限”標志顏色）

sqlfiddle

構建查詢

create function buildQuery() returns varchar(4000) 
not deterministic 
reads sql data 
begin 
  -- variables
  declare query varchar(4000);
  declare maxcols int;
  declare counter int;

  -- initialize
  set query   = '';
  set maxcols = 0;
  set counter = 0;

  -- get the max amount of columns
  select count(color_id) as maxflagcolors into maxcols 
  from flag_colors 
  group by flag_id 
  order by maxflagcolors desc limit 1;

  -- build the query
  while counter < maxcols do
    set counter = counter + 1;
    set query=concat(query,',replace(substring(substring_index(group_concat(c.color), '','',', counter,'),length(substring_index(group_concat(c.color),'','',', counter,'-1)) + 1),'','','''') as color' ,counter);        
  end while;

  -- return
  return query;
end//

---

運行查詢

set @q = buildQuery();

set @q = concat(
'SELECT 
    f.flag ', @q, '
FROM 
    flags f
LEFT JOIN
    flag_colors fc
    ON fc.flag_id = f.id
LEFT JOIN
    colors c
    ON c.id = fc.color_id
GROUP BY f.id');


prepare s from @q;
execute s;
deallocate prepare s;

---

結果

sqlfiddle