In MySQL, there's a common rule that you can't modify the same table that you use in the SELECT part of a query. This behaviour is documented in the MySQL documentation and is intended to maintain data integrity and prevent potential conflicts that could arise when modifying a table that's being used in a query.
The reason why this rule exists is because MySQL executes an UPDATE statement in two phases: first it reads the data that needs to be updated and then it modifies the table. If the same table is used in both the SELECT and UPDATE parts of the query, it can cause the SELECT statement to return a different set of data than what the UPDATE statement is actually modifying. This can lead to unexpected results and potentially corrupt data.
While this rule is in place for good reason, there are situations where you may need to modify a table that's being used in a SELECT statement. In such cases, you can use multiple subselects to work around the limitation.
For example, consider the following query:
UPDATE mytable SET col1 = 'value' WHERE col2 = (SELECT MAX(col2) FROM mytable);This query violates the rule since it's attempting to modify the same table that's being used in the SELECT part of the statement. To work around this limitation, you can use multiple subselects:
UPDATE mytable SET col1 = 'value' WHERE col2 = (SELECT max_col2 FROM (SELECT MAX(col2) AS max_col2 FROM mytable) AS t);In this updated query, the SELECT statement that retrieves the maximum value of col2 is moved into a subselect, and then that subselect is moved into another subselect that's used in the UPDATE statement. This allows the query to work without violating the rule.
In summary, while you can't modify the same table you use in the SELECT part of a query in MySQL, you can work around this limitation by using multiple subselects. By following this rule and using the workaround when needed, you can help maintain data integrity and ensure your SQL queries are behaving as expected.
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