How Can I Perform Conditional Joins in MySQL Based on an Enumerated Type?

Conditional Joins in MySQL: Dynamically Joining Tables Based on an Enumerated Type

Problem:

Consider a MySQL table schema with columns id1, id2, and type. The type column is an enumerated value that specifies the name of another table. The task is to perform a join with the specified table based on the value of type.

For example, you want to perform the following conditional joins:

• If type is 'table1', join table1
• If type is 'table2', join table2

Solution:

While MySQL does not support conditional joins directly, one workaround is to use a multi-table left join with a case operator:

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'

In this query:

• The main table t is joined with t2 and t3.
• The condition in the LEFT JOIN ensures that the join is performed only if the type column matches the corresponding table name.
• The resulting rows contain all the columns from the t table, along with the corresponding columns from the joined tables (id2 from t2 and id3 from t3) if the join condition is met.

Note:

This solution requires all potential joined tables to exist in the database, regardless of the value of type.

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