本文探讨了 SQL 中 DELETE 语句的有效用法，并通过一个具体例子解释了如何正确使用带有子查询的 DELETE 语句。文章指出有效的 DELETE 语句应该确保子查询只返回单一记录，以避免数据完整性问题。
QUESTION NO: 49
B. DELETE * FROM employeesWHERE employee_id=(SELECT employee_id FROM
new_employees);
C. DELETE FROM employeesWHERE employee_id IN (SELECT employee_id FROM
new_employees WHERE name = ‘Carrey’);
D. DELETE * FROM employeesWHERE employee_id IN (SELECT employee_id FROM
new_employees WHERE name = ‘Carrey’);
Answer: C
Examine the structure of the EMPLOYEES and NEW_EMPLOYEES tables:
Which DELETE statement is valid?
A. DELETE FROM employeesWHERE employee_id = (SELECT employee_id FROM employees);B. DELETE * FROM employeesWHERE employee_id=(SELECT employee_id FROM
new_employees);
C. DELETE FROM employeesWHERE employee_id IN (SELECT employee_id FROM
new_employees WHERE name = ‘Carrey’);
D. DELETE * FROM employeesWHERE employee_id IN (SELECT employee_id FROM
new_employees WHERE name = ‘Carrey’);
Answer: C
Explanation:
delete * from 语法错误 BD错误
子查询中必须为返回单条的记录，所以C正确。
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