SELECTcolumn_name,IF(column_nameISNULL,'Empty','Not Empty')ASstatusFROMtable_name; 使用CASE语句检查列是否为空: 代码语言:sql AI代码解释 SELECTcolumn_name,CASEWHENcolumn_nameISNULLTHEN'Empty'ELSE'Not Empty'ENDASstatusFROMtable_name; 在这些查询中,我们使用IF和CASE语句来根据列的值返回相应的结果,以...
CREATE TABLE `t_student` ( `no` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) DEFAULT NULL, `cno` int(11) DEFAULT NULL, PRIMARY KEY (`no`), KEY `cno` (`cno`), CONSTRAINT `t_student_ibfk_1` FOREIGN KEY (`cno`) REFERENCES `t_class` (`classno`) ) ENGINE=InnoDB AUTO...
SELECT column_name, IF(column_name IS NULL, 'Empty', 'Not Empty') AS status FROM table_name; 1. 使用CASE语句检查列是否为空: SELECT column_name, CASE WHEN column_name IS NULL THEN 'Empty' ELSE 'Not Empty' END AS status FROM table_name; 1. 2. 3. 4. 5. 6. 在这些查询中,我们...
IS NULL和IS NOT NULL条件只能用于检测NULL值,而空字符串需要使用=或<>等条件进行检测。 解决方法: 代码语言:txt 复制 -- 查询字段为NULL的记录 SELECT * FROM table_name WHERE column_name IS NULL; -- 查询字段不为空字符串的记录 SELECT * FROM table_name WHERE column_name <> ''; 问题2:为什么在...
(root@localhost mysql3306.sock)[zlm]>select * from test_null where name is not null;+---+---+| id | name |+---+---+| 1 | zlm |+---+---+1 row in set (0.00 sec)(root@localhost mysql3306.sock)[zlm]>select * f...
test_nullwherename=null;23Emptyset(0.00sec)2425(root@localhostmysql3306.sock)[zlm]>select*fromtest_nullwherenameisnull;26+---+---+27|id|name|28+---+---+29|2|NULL|30+---+---+311rowinset(0.00sec)3233(root@localhostmysql3306.sock)[zlm]>select*fromtest_nullwherenameisnotnull;34+...
ⅠMySQL约束条件 【一】什么是约束条件 约束条件:限制表中的数据,保证添加到数据表中的数据准确和可靠性!凡是不符合约束的数据,插入时就会失败! 约束条件在创建表时可以使用, 也可以修改表的时候添加约束条件 【二】约束条件概览 null 和 not null 为空和不为空 限制
Empty set (0.00 sec) (root@localhost mysql3306.sock)[zlm]>select * from test_null where name is null; +---+---+ id | name | +---+---+ 2 | NULL | +---+---+ 1 row in set (0.00 sec) (root@localhost mysql3306.sock)[zlm]>select * from test_null where name is not ...
通常能听到的答案是使用了NULL值的列将会使索引失效,但是如果实际测试过一下,你就知道IS NULL会使用索引.所以上述说法有漏洞.着急的人拉到最下边看结论 Preface Null is a special constraint of columns.The columns in table will be added null constrain if you do not define the column with “not null”...
In MariaDB 10.4, all the columns in my table are described as 'not null'. When I execute a query such as 'select * from XXX where <column name> is not null', the results include all rows in that column, whether they contain data or are empty. The 'desc ' query returns: +---...