SQL join + group_concat 不返回一些行

Question

在我的数据库中，我有以下表格：

People
+-----------+------------+
| IdPeople  | Name       |
+-----------+------------+
|         1 | James      |
|         2 | Chris      |
+-----------+------------+

ref
+---------+-------------+------+
| People  | Color       | Code |
+---------+-------------+------+
|       1 |           2 |    1 |
|       1 |           1 |    2 |
|       1 |           6 |    3 |
|       2 |           1 |    1 |
|       2 |           6 |    4 |
|       2 |           4 | NULL |
|       2 |           5 | NULL |
+---------+-------------+------+

Colors
+--------+--------------------+
| IdCol  | Color              |
+--------+--------------------+
|      1 | Blue               |
|      2 | Green              |
|      3 | Yellow             |
|      4 | Red                |
|      5 | Black              |
|      6 | White              |
+--------+--------------------+

Codes
+--------+----------------+
| IdCode | Code           |
+--------+----------------+
|      1 | C++            |
|      2 | JavaScript     |
|      3 | Python         |
|      4 | HTML           |
+--------+----------------+

我想加入所有的表来得到这样的东西：

+------------+---------------------------+------------------------------------+
| Name       | Color                     | Code                               |
+------------+---------------------------+------------------------------------+
| Chris      | Blue, White, Red, Black   | C++, HTML                          |
| James      | Green, Blue, White        | C++, JavaScript, Python            |
+------------+---------------------------+------------------------------------+

我试着像这样加入 group_concat

SELECT People.Name,
       group_concat(Colors.Color separator ", ") AS "Color",
       group_concat(Codes.Code separator ", ") AS "Code"
FROM People
INNER JOIN ref ON People.IdPeople=ref.People
INNER JOIN Colors ON ref.Color=Colors.IdCol
INNER JOIN Codes ON ref.Code=Codes.Code
GROUP BY Name;

但是，由于 group_concat 不返回空行，因此我在 Chris 行中获得了除 Black 和 White 之外的所有内容。 我不知道该怎么做，所以有人可以帮助我吗？

Answer 1

由于ref.code可以为 null，您必须外部LEFT OUTER JOIN Codes ON ...代码表： LEFT OUTER JOIN Codes ON ... 。

完整查询：

SELECT People.Name,
       group_concat(Colors.Color separator ", ") AS "Color",
       group_concat(Codes.Code separator ", ") AS "Code"
FROM People
INNER JOIN REF ON People.IdPeople=ref.People
INNER JOIN Colors ON ref.Color=Colors.IdCol
LEFT JOIN Codes ON ref.Code=Codes.Code
GROUP BY Name;