MySQL 连接表，其中表名是另一个表的字段

Question

我有5张桌子。 1 个主要的和 4 个附加的（它们有不同的列）。

1. 对象
2. obj_mobiles
3. obj_tablets
4. obj_computers

这是我的主表（对象）的结构。

ID |类型 |名称 |等等...

所以我想做的是将对象与其他（obj_mobiles，obj_tablets，...）表连接起来，具体取决于类型字段。 我知道我应该使用动态 SQL。但我无法办理手续。我认为它应该看起来像这样。

SELECT objects.type into @tbl FROM objects;
PREPARE stmnt FROM "SELECT * FROM objects AS object LEFT JOIN @tbl AS info ON object.id = info.obj_id"; 
EXECUTE stmnt;
DEALLOCATE PREPARE stmnt;

Aslo 伪代码

SELECT * FROM objects LEFT JOIN [objects.type] ON ... 

任何人都可以发布程序吗？另外我想要所有行而不仅仅是 1 行。 谢谢。

Answer 1

如果您想要所有行（批量输出）而不是一次一行，那么下面的代码应该很快，并且所有行的输出将包含所有列。

让我们考虑下面的表字段。
obj_mobiles - ID | obj_mobiles M1 | M2
obj_tablets-ID | obj_tablets T1 | T2
obj_计算机- ID | C1 | C2
对象- ID |类型 |名称 |等等，

Select objects.*, typestable.*
from (
    select ID as oID, "mobile" as otype, M1, M2, NULL T1, NULL T2, NULL C1, NULL C2 from obj_mobiles 
    union all
    select ID as oID, "tablet" as otype, NULL, NULL, T1, T2, NULL, NULL from obj_tablets 
    union all
    select ID as oID, "computer" as otype, NULL, NULL, NULL, NULL, C1, C2 from obj_computers) as typestable 
        left join objects on typestable.oID = objects.ID and typestable.otype = objects.type;

+------+--------------------+----------+------+----------+------+------+------+------+------+------+
| ID   | name               | type     | ID   | type     | M1   | M2   | T1   | T2   | C1   | C2   |
+------+--------------------+----------+------+----------+------+------+------+------+------+------+
|    1 | Samsung Galaxy s2  | mobile   |    1 | mobile   |    1 | Thin | NULL | NULL | NULL | NULL |
|    2 | Samsung Galaxy Tab | tablet   |    2 | tablet   | NULL | NULL | 0.98 |   10 | NULL | NULL |
|    3 | Dell Inspiron      | computer |    3 | computer | NULL | NULL | NULL | NULL | 4.98 | 1000 |
+------+--------------------+----------+------+----------+------+------+------+------+------+------+

表格创建如下。

mysql> create table objects (ID int, name varchar(50), type varchar (15));
Query OK, 0 rows affected (0.05 sec)
mysql> insert into objects values (1, "Samsung Galaxy s2", "mobile"), (2, "Samsung Galaxy Tab", "tablet"), (3, "Dell Inspiron", "computer");
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0


mysql> create table obj_mobiles (ID int, M1 int, M2 varchar(10));
Query OK, 0 rows affected (0.03 sec)
mysql> insert into obj_mobiles values (1, 0.98, "Thin");
Query OK, 1 row affected (0.00 sec)


mysql> create table obj_tablets (ID int, T1 float, T2 int(10));
Query OK, 0 rows affected (0.03 sec)
mysql> insert into obj_tablets values (2, 0.98, 10);
Query OK, 1 row affected (0.00 sec)


mysql> create table obj_computers (ID int, C1 float, C2 int(10));
Query OK, 0 rows affected (0.03 sec)
insert into obj_computers values (3, 4.98, 1000);

还要确认列的数据类型与原始列相同，结果将保存到表中并在下面检查数据类型。

create table temp_result as
Select objects.*, typestable.*
from (
    select ID as oID, "mobile" as otype, M1, M2, NULL T1, NULL T2, NULL C1, NULL C2 from obj_mobiles 
    union all
    select ID as oID, "tablet" as otype, NULL, NULL, T1, T2, NULL, NULL from obj_tablets 
    union all
    select ID as oID, "computer" as otype, NULL, NULL, NULL, NULL, C1, C2 from obj_computers) as typestable 
        left join objects on typestable.oID = objects.ID and typestable.otype = objects.type;

mysql> desc temp_result;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(50) | YES  |     | NULL    |       |
| type  | varchar(15) | YES  |     | NULL    |       |
| oID   | int(11)     | YES  |     | NULL    |       |
| otype | varchar(8)  | NO   |     |         |       |
| M1    | int(11)     | YES  |     | NULL    |       |
| M2    | varchar(10) | YES  |     | NULL    |       |
| T1    | float       | YES  |     | NULL    |       |
| T2    | int(11)     | YES  |     | NULL    |       |
| C1    | float       | YES  |     | NULL    |       |
| C2    | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
11 rows in set (0.00 sec)

Answer 2

如果 objects 是父表，则意味着 objects.ID 是唯一的对象。正确的？所有其他项目（手机、平板电脑、计算机）都是对象的子项目，并且手机和平板电脑不能具有相同的 ID。如果是这样，使用这个简单的查询就足够了 -

SELECT * FROM objects o
  LEFT JOIN obj_mobiles m
    ON o.id = m.ID
  LEFT JOIN obj_tablets t
    ON o.id = t.ID
  LEFT JOIN obj_computers c
    ON o.id = c.ID

添加 WHERE 子句来过滤类型，例如：WHERE o.type = 'mobile'。

Answer 3

使用 CONCAT 创建准备语句。
例如。

@sql = CONCAT('SELECT * FROM objects AS object LEFT JOIN ', @tbl, ' AS info ON object.id = info.obj_id');
PREPARE stmnt FROM @sql;
...