如果为空则更新列A，否则更新列B，否则如果两个列都不为空，则对MySQL不执行任何操作
为此，请使用IF()ISNULL属性。让我们首先创建一个表-
mysql> create table DemoTable1976 ( FirstName varchar(20), LastName varchar(20) );
使用插入命令在表中插入一些记录-
mysql> insert into DemoTable1976 values('John','Doe');
mysql> insert into DemoTable1976 values('John',NULL);
mysql> insert into DemoTable1976 values(NULL,'Miller');
mysql> insert into DemoTable1976 values('Chris','Brown');使用select语句显示表中的所有记录-
mysql> select * from DemoTable1976;
这将产生以下输出-
+-----------+----------+ | FirstName | LastName | +-----------+----------+ | John | Doe | | John | NULL | | NULL | Miller | | Chris | Brown | +-----------+----------+ 4 rows in set (0.00 sec)
这是如果为null则更新一列的查询，否则更新另一列，否则如果两列都不为null则不执行任何操作-
mysql> update DemoTable1976 set FirstName=if(FirstName IS NULL,'David',FirstName), LastName=if(LastName IS NULL,'Brown',LastName); Rows matched: 4 Changed: 2 Warnings: 0
让我们再次检查表记录-
mysql> select * from DemoTable1976;
这将产生以下输出-
+-----------+----------+ | FirstName | LastName | +-----------+----------+ | John | Doe | | John | Brown | | David | Miller | | Chris | Brown | +-----------+----------+ 4 rows in set (0.00 sec)