How to Workaround the \'Can\'t Reopen Table\' Error in MySQL Temporary Tables?

Tackling the "Can't reopen table" Error in MySQL

In data processing, the ability to utilize a table multiple times in a complex query is often essential. However, when working with temporary tables in MySQL, this seemingly basic operation encounters the frustrating "Can't reopen table" error.

Background

As described by Kris, a complex query involving multiple INNER JOINS on a temporary table results in the error. This occurs because MySQL does not allow temporary tables to be opened multiple times within the same query.

Workaround

To navigate this limitation, a simple yet effective solution is to duplicate the temporary table, essentially creating two identical versions. This works particularly well when the temporary table is relatively small, as is often the case.

Implementation

The following steps outline the implementation of this workaround:

1. Create the temporary table:

CREATE TEMPORARY TABLE search (baseID INT, condition TEXT);

2. Fill the temporary table with data:

INSERT INTO search (baseID, condition) VALUES (1, 'condition1'), (1, 'condition2'), ...;

3. Now, duplicate the temporary table:

CREATE TEMPORARY TABLE search_copy AS SELECT * FROM search;

4. You can now use the duplicated table as many times as necessary in your complex query:

SELECT
  *
FROM search
INNER JOIN search_copy f1 ON f1.baseID = search.baseID AND f1.condition = condition1
INNER JOIN search_copy f2 ON f2.baseID = search.baseID AND f2.condition = condition2
...
INNER JOIN search_copy fN ON fN.baseID = search.baseID AND fN.condition = conditionN;

Benefits

This workaround offers several advantages:

• Simplicity: It is straightforward to implement, even for junior developers.
• Effectiveness: It eliminates the "Can't reopen table" error, allowing for complex queries with temporary tables.
• Space efficiency: Duplicating the temporary table does not consume additional disk space on the server.

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