如果用于在 UPDATE 语句的 SET 子句中分配新值的子查询没有返回任何行，MySQL 将返回什么?
mysqlmysqli database
在这种情况下，MySQL 将为 SET 子句提供一个 NULL 值。以下示例将演示这一点 −
示例
mysql> Select * from student; +----+---------+-----------+ | Id | Name | grade | +----+---------+-----------+ | 1 | Rahul | Good | | 2 | Gaurav | Good | | 3 | Raman | Excellent | | 4 | Harshit | Average | | 5 | Aarav | Best | | 6 | Ram | average | +----+---------+-----------+ 6 rows in set (0.00 sec) mysql> select * from info; +------+-----------+ | id | remarks | +------+-----------+ | 1 | Good | | 2 | Good | | 3 | Excellent | | 4 | Average | | 5 | Best | +------+-----------+ 5 rows in set (0.00 sec)
从以上两个表中，我们可以看到‘student’表有 6 行，‘grade’的值为‘average’，其中 id = 6，而‘info’表有 5 行。现在，以下查询中使用的子查询将不会提供任何行来在 SET 子句中提供新值，因此它将插入一个 NULL 值。可以从以下查询的输出中进行检查 −
mysql> UPDATE STUDENT SET grade = (SELECT remarks from info WHERE info.id = student.id) WHERE id = 6; Query OK, 1 row affected (0.04 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> Select * from student; +----+---------+-----------+ | Id | Name | grade | +----+---------+-----------+ | 1 | Rahul | Good | | 2 | Gaurav | Good | | 3 | Raman | Excellent | | 4 | Harshit | Average | | 5 | Aarav | Best | | 6 | Ram | NULL | +----+---------+-----------+ 6 rows in set (0.00 sec)
