Why Do I Get 'Table is Specified Twice' Error in MySQL UPDATE Queries?

Error: "Table is specified twice, both as a target for 'UPDATE' and as a separate source for data"

When executing MySQL queries that involve updating a table and referencing it as a data source within the same query, you may encounter the error "Table is specified twice." This error occurs when the same table is both the target of the UPDATE statement and appears as a separate table in the subquery used to determine the update criteria.

To resolve this error and successfully update the desired table, follow these steps:

1. Extract the subquery: Identify the subquery within the UPDATE statement that is causing the issue. In the provided example, the subquery is:

SELECT m2.branch_id FROM manager AS m2 WHERE (m2.branch_id, m2.year) IN (...)

2. Create a derived table: Create a derived table by wrapping the subquery in another SELECT statement and assigning an alias to it. This will allow you to reference the subquery without explicitly specifying the manager table twice.

SELECT * FROM (SELECT m2.branch_id FROM manager AS m2 WHERE (m2.branch_id, m2.year) IN (...)) AS subquery

3. Use the derived table: Replace the original subquery with the derived table in the UPDATE statement.

UPDATE manager
SET status = 'Y'
WHERE branch_id IN (
    SELECT branch_id
    FROM subquery
);

By using a derived table, you avoid specifying the manager table as both a target and data source, resolving the "Table is specified twice" error. This technique ensures that the subquery is only referenced once and allows the UPDATE statement to execute successfully.

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