How to Resolve the 'Table Specified Twice' Error in MySQL When Using Subqueries?

MySQL Error: Table Specified Twice

When attempting to update a table while also using it as a data source, MySQL may issue an error indicating that the table is specified twice. This typically occurs when using subqueries with table aliasing.

For instance, consider the following query:

UPDATE manager AS m1
SET m1.status = 'Y'
WHERE m1.branch_id IN (
  SELECT m2.branch_id
  FROM manager AS m2
  WHERE (m2.branch_id, m2.year) IN (
    SELECT DISTINCT branch_id, year
    FROM branch_master
    WHERE type = 'finance'
  )
);

This query attempts to update the manager table based on a subquery that checks whether certain fields in manager match values from the branch_master table where the type is 'finance'. However, MySQL objects to the fact that the manager table is used both as the target (in the UPDATE clause) and as a data source (in the subquery).

To resolve this issue and prevent the "Table is specified twice" error, we can use a derived table for the subquery. This involves creating a temporary table based on the original table, then selecting from that derived table:

FROM (SELECT * FROM manager) AS m2

By enclosing the original manager table in parentheses and aliasing the result as m2, we create a new derived table. This allows us to select data from the manager table without explicitly specifying the manager name twice.

Updated Query:

UPDATE manager
SET status = 'Y'
WHERE branch_id IN (
  SELECT branch_id
  FROM (SELECT * FROM manager) AS m2
  WHERE (m2.branch_id, m2.year) IN (
    SELECT branch_id, year
    FROM branch_master
    WHERE type = 'finance'
  )
);

This updated query correctly uses a derived table for the subquery, resolving the "Table is specified twice" error and allowing the query to execute successfully.

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