//In MySQL,null means false,too. It truns null as a result if any expression contains null value. 任何有返回值的表达式中有NULL参与时,都会得到另外一个NULL值。 (root@localhost mysql3306.sock)[zlm]>select ifnull(null,'First is null'),ifnull(null+10,'First is null'),ifnull(concat('ab...
在mysql中0和空值,空格,tab,回车都是相等的,至于mysql为什么这么设定,我也不知道。所以如果不想让0和这些值相等,可以把字段设置成varchar类型,就不会有这些问题啦! 3.mysql中的常用内置函数: 因为刚使用mysql,对于其内置函数不是很熟悉,下面copy了一下mysql内置的一些函数,供和我一样初涉mysql的小伙伴学习。 ①...
使用show create database命令可以查看创建数据库时所使用的语句以及字符集,可以看到数据库mysql和redmine均未指定charset创建数据库,所以为缺省字符集latin1. mysql> show create database mysql; +---+---+ | Database | Create Database | +---+---+ | mysql | CREATE DATABASE `mysql` /*!40100 DEFA...
SQL pattern matching enables you to use _ to match any single character and % to match an arbitrary【[ˈɑːrbɪtreri] 任意的;】 number of characters (including zero characters). In MySQL, SQL patterns are case-insensitive by default. Some examples are shown here. Do not use = or ...
NULL |1> NULL |15+---+---+---+---+16| NULL | NULL | NULL | NULL |17+---+---+---+---+181rowinset (0.00sec)1920//It cannot be compared with number.21//In MySQL,null means false,too. It truns null as a result if any expression contains null value. 1(root@localhost...
data type cannot handle NULL values. As such, when assigning values from a query to a DateTime variable, you must first check whether the value is in fact NULL. When using a MySqlDataReader, use the .IsDBNull method to check whether a value is NULL before making the assignment: C#...
MySql.Data.MySqlClient.MySqlDataAdapter DA = new MySql.Data.MySqlClient.MySqlDataAdapter("select ifnull(null,curdate() - interval 4 day);", Conn); System.Data.DataTable DT = new System.Data.DataTable(); DA.Fill(DT); Conn.Close(); Console.WriteLine("Field type: '" + DT.Rows[...
MySQL中的NULL和空字符串('')是两种不同的概念: NULL:表示一个字段没有值,或者值未知。它是一个特殊的标记,不同于任何其他值,甚至不同于空字符串。 空字符串:表示字段有一个长度为零的字符串值。 相关优势 NULL的优势: 可以表示数据的缺失或未知状态。 在某些情况下,NULL可以节省存储空间,因为数据库可能会...
Partitioning in MySQL does nothing to disallow NULL as the value of a partitioning expression, whether it is a column value or the value of a user-supplied expression. Even though it is permitted to use NULL as the value of an expression that must otherwise yield an integer, it is importan...
Bug #49936Comparing NULLABLE type for NULL value in database Submitted:25 Dec 2009 7:49Modified:26 May 2011 17:21 Reporter:fahad khalilEmail Updates: Status:Not a BugImpact on me: None Category:Connector / NETSeverity:S1 (Critical)