SQL: How can we make a table1 JOIN table2 ON a table given in a field in table1?

问题

Imagine I have table1 which has a column named 'table_name'. I use table1.table_name to store the name of another table in the database. The referenceable tables would all have a field 'target_id.

Is is possible to use table_name in a JOIN statement?

For example:

SELECT t1.*, t2.* FROM table1 AS t1
                  JOIN table1.table_name AS t2 ON t1.table1_id = t2.target_id

The obvious solution is to use the script (C++ in my case) to get the table name first, and construct a SQL query from it. The question is: can we bypass the script and do this directly in SQL (MySQL)?

Edit: What is dynamic SQL?

回答1:

The only chance you have is to do 2 SQL statements:

select the tablename you need
use this table-name to dynamically build the secound query to get the data you need - what you want isn't possible to do with SQL directly (and it sounds like you've designed your database wrong in some way - but that's hard to say without knowing what's the goal of it).

回答2:

I know I'm late to the party, but I wanted to offer a different solution. I see this sort of thing a lot in audit tables. The column table_name would refer to "what table was changed" and table1_id would refer to the ID of the row that changed in that table. In this case, the audit table is pointing back to many different tables that don't normally get joined.

Here goes:

SELECT t1.*, t2.*, t3.*, t4.*, t5.*

FROM table1 AS t1

left JOIN table2 AS t2 
    ON t1.table1_id = t2.target_id
    and t1.table_name = 'table2'

left JOIN table3 AS t3
    ON t1.table1_id = t3.target_id
    and t1.table_name = 'table3'

left JOIN table4 AS t4 
    ON t1.table1_id = t4.target_id
    and t1.table_name = 'table4'

left JOIN table5 AS t5 
    ON t1.table1_id = t5.target_id
    and t1.table_name = 'table5'

Of course, the main drawback is that each table that can be possibly referenced needs to be explicitly included in the SQL command.

You can get more elegant output using this as your select list:

SELECT 
     t1.*, 
     coalesce(t2.fieldA, t3.fieldA, t4.fieldA, t5.fieldA) as fieldA,
     coalesce(t2.fieldB, t3.fieldB, t4.fieldB, t5.fieldB) as fieldB

etc

来源：https://stackoverflow.com/questions/4164808/sql-how-can-we-make-a-table1-join-table2-on-a-table-given-in-a-field-in-table1

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mysql

sql

dynamic-sql