MySQL 5.7 error (1093: You can't specify targe

问题:

I have a table 'employees' and I'm trying to set some attributes (e.g. salary) to the same value as some other value in the table. My understanding of this error is that it can be avoided with the following workaround, using a temporary table:

UPDATE employees
SET salary=(SELECT salary FROM (SELECT * FROM employees WHERE employee_id= '123') AS t1)
WHERE employee_id='456';

However, I am still getting the same error code ("can't specify target table 'employees' for update in FROM clause") when I try this. Is there some other issue here?

回答1:

The issue is a functional change in mysql 5.7,scroll towards the end

The optimizer now handles derived tables and views in the FROM clause in consistent fashion to better avoid unnecessary materialization and to enable use of pushed-down conditions that produce more efficient execution plans. However, for statements such as DELETE or UPDATE that modify tables, using the merge strategy for a derived table that previously was materialized can result in an ER_UPDATE_TABLE_USED error:

Either use the JOIN or force the otimizer to behave like in the previous version with:

SET optimizer_switch = 'derived_merge=off';