选择使用动态生成的表名 [英] Select using dynamically generated tablename

问题描述

我遇到以下问题.我的表(geo_table)结构如下:

I've got the following problem. My table (geo_table) structure is as follows:

foreign_table_id | foreign_table_name | some_other_fields...

foreign_table_name存储不同表的名称，而foreign_table_id存储这些表中的ID.我想做的是检查上述表中的行是否存储了对其他表的有效引用.因此，我需要从外部表中选择一行，其名称基于存储在foreign_table_name中的字符串.我的代码无法正常工作，因为它存在语法错误，但可以清楚地表达出我想要实现的目标:

foreign_table_name stores names of different tables and foreign_table_id stores ids within these tables. What I want to do is check whether a row in the above mentioned table stores valid reference to some other table. Thus I need to select a row from foreign table which name is based on string stored in foreign_table_name. My code didn't work because it has syntax error but it clearly presents what I wanted to achieve:

SELECT foreign_table_id, foreign_table_name FROM `geo_table` gt
WHERE (
    SELECT COUNT(*) FROM gt.foreign_table_name AS t
    WHERE gt.foreign_table_id = t.uid
) > 0

推荐答案

如果您知道所有可能的表名，则可以使用条件语法来实现它:

If you know all possible table names then you can implement it using conditional syntax:

SELECT foreign_table_id, foreign_table_name FROM `geo_table` gt
WHERE 
    CASE gt.foreign_table_name
        WHEN 'table1' THEN 
            EXISTS (
                SELECT * FROM table1
                WHERE id = gt.foreign_table_id
            )
        WHEN 'table2' THEN 
            EXISTS (
                SELECT * FROM table2
                WHERE id = gt.foreign_table_id
            )
        ELSE
            FALSE
    END

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