How Can I Perform Conditional Joins Based on Enumerated Table Names in MySQL?

Conditional Joins in MySQL

When dealing with tables that contain an enumerated value representing the table name of another table, it becomes necessary to perform a join based on the type value. This poses a unique challenge in MySQL, as it doesn't support dynamic joins.

However, a workaround can be achieved using case expressions and multiple joins.

A simple example would be a table with columns id1, id2, and type. The type column contains the name of another table. To join the type table based on the value of type, one could use the following approach:

SELECT
t.id,
t.type,
t2.id AS id2,
t3.id AS id3

FROM
t

LEFT JOIN
t2 ON t2.id = t.id AND t.type = 't2'

LEFT JOIN
t3 ON t3.id = t.id AND t.type = 't3'

This query will perform a left join on the t2 and t3 tables based on the value of type. If the type value matches t2, it will return the id column from t2 as id2. Similarly, if the type value matches t3, it will return the id column from t3 as id3.

While not as elegant as a dynamic join, this method allows for conditional joins based on enumerated table names in MySQL.

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