修改值
基本语法
update **table_name** set **col_name1**=expr1,[col_name2=expr1] where where_condition
实例
修改分数低于60的人的分数到59
MariaDB [istester]> select * from idoxu where grade < 60;
+----+--------+----------+----------+-------+
| id | stu_id | c_name | istester | grade |
+----+--------+----------+----------+-------+
| 6 | 33 | istester | NULL | 20 |
+----+--------+----------+----------+-------+
1 row in set (0.00 sec)
MariaDB [istester]> select * from idoxu;
+----+--------+----------+----------+-------+
| id | stu_id | c_name | istester | grade |
+----+--------+----------+----------+-------+
| 4 | 11 | idoxu | NULL | 90 |
| 5 | 12 | lin | NULL | 100 |
| 6 | 33 | istester | NULL | 20 |
+----+--------+----------+----------+-------+
3 rows in set (0.00 sec)
MariaDB [istester]> update idoxu set grade=59 where grade < 60;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1 Changed: 1 Warnings: 0
MariaDB [istester]> select * from idoxu;
+----+--------+----------+----------+-------+
| id | stu_id | c_name | istester | grade |
+----+--------+----------+----------+-------+
| 4 | 11 | idoxu | NULL | 90 |
| 5 | 12 | lin | NULL | 100 |
| 6 | 33 | istester | NULL | 59 |
+----+--------+----------+----------+-------+
3 rows in set (0.00 sec)
修改性别值为空的数据为0
MariaDB [istester]> select * from istester where sex is NULL;
+----+--------+------+-------+------------+---------+-------+
| id | uname | sex | birth | department | address | idoxu |
+----+--------+------+-------+------------+---------+-------+
| 1 | idoxu | NULL | NULL | NULL | NULL | 2020 |
| 2 | idoxu2 | NULL | NULL | NULL | NULL | 2020 |
+----+--------+------+-------+------------+---------+-------+
2 rows in set (0.00 sec)
MariaDB [istester]> update istester set sex=0 where sex is null;
Query OK, 2 rows affected (0.03 sec)
Rows matched: 2 Changed: 2 Warnings: 0
MariaDB [istester]> select * from istester;
+----+--------+------+-------+------------+---------+-------+
| id | uname | sex | birth | department | address | idoxu |
+----+--------+------+-------+------------+---------+-------+
| 1 | idoxu | 0 | NULL | NULL | NULL | 2020 |
| 2 | idoxu2 | 0 | NULL | NULL | NULL | 2020 |
| 11 | idoxu3 | 1 | NULL | NULL | NULL | NULL |
| 12 | idoxu4 | 2 | NULL | NULL | NULL | NULL |
| 13 | idoxu5 | 1 | NULL | NULL | NULL | NULL |
| 14 | idoxu6 | 2 | NULL | NULL | NULL | NULL |
+----+--------+------+-------+------------+---------+-------+
6 rows in set (0.00 sec)
注意:mysql中值的判断有=,<,>,<=,>=,但是针对空值的情况下不能用=NULL去判断，NULL专门有个关键字is去判断
