MySQL联接表，其中表名是另一个表的字段 [英] MySQL join tables where table name is a field of another table

问题描述

我有5张桌子.一个主数据库和另外四个主数据库(它们具有不同的列).

I have 5 tables. One primary and 4 additional (they have different columns).

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

这是我的主表(对象)的结构.

Here is the structure of my main table (objects).

ID |类型名称|等等...

ID | type | name | etc...

所以我想做的就是根据类型字段将对象与其他(obj_mobiles，obj_tablets，...)表联接. 我知道我应该使用动态SQL.但我无法制定程序.我认为它应该看起来像这样.

So what I want to do is to join objects with other (obj_mobiles,obj_tablets,...) tables, depending on type field. I know that I should use dynamic SQL. But I can't make procedure. I think it should look like something like this.

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?我也想让所有的行不只是1行. 谢谢.

Can anyone post procedure? Also I want to have all rows not just 1 row. Thanks.

推荐答案

如果您希望所有行(批量输出)而不是一次仅输出一行，则下面的内容应该很快，并且所有行的输出将包含所有列

If you want all rows (bulk output) and not one row at a time, the below should be fast and also the output for all rows will contain all columns.

让我们在下面将其视为表的字段. obj_mobiles-ID | M1 | M2 obj_tablets-ID | T1 | T2 obj_computers-ID | C1 | C2 对象-ID |类型名称|等等，

Lets consider below to be the fields of the tables. obj_mobiles - ID | M1 | M2 obj_tablets - ID | T1 | T2 obj_computers - ID | C1 | C2 objects - ID | type | name | etc.,

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 |
+------+--------------------+----------+------+----------+------+------+------+------+------+------+

该表如下创建.

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，将结果保存到表中并在下面检查数据类型.

also to confirm the datatypes of the columns are same as the original columns, the result is saved into a table and datatypes are checked below.

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)

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