字符比较 匹配空 非空

字符比较/匹配空/非空

字符比较时，字段类型必须是字符类型
where 字段名 符号 "字符串"

= 相等
！= 不相等
is null 匹配空
is not null 非空

mysql> select name from user where name="bob"; 如果不在对应目录下，那么要写完整路径 库名.表名 ，如db3.user。
mysql> select name from user where shell!="/bin/bash";
mysql> select id,name from user where uid is null;
mysql> select id,name,uid from user where uid is not null;