search
HomeDatabaseMysql TutorialMysql中IFNULL与IN操作_MySQL

bitsCN.com

Mysql IFNULL操作

项目中用到的,当SQL查询某个字段为空的时候,查询结果中设置其值为默认值。最笨的方法当然是对查询结果进行处理了,遍历查询结果,当为空的时候,设置其值:

 代码如下 复制代码
$len=count($result);
for($i=0;$i    $var = $result[$i]['name'];
 if(!$var){
  $result[$i]['name']='default_name';
 }
}
 

如上办法,不仅费时,还??隆H绻?茉?ql语句中直接解决多好啊。

SQL中有ISNULL方法,介绍如下:


ISNULL

使用指定的替换值替换 NULL。

语法

ISNULL ( check_expression , replacement_value )

参数

check_expression

将被检查是否为 NULL的表达式。check_expression 可以是任何类型的。
replacement_value

在 check_expression 为 NULL时将返回的表达式。replacement_value 必须与 check_expresssion 具有相同的类型。

 
但在Mysql中,isnull只是用来判断是否为空,不能实现替换功能,照上面写的话,会直接报错(Incorrect parameter count in the call to native function 'isnull' Errornumber:1582 )。

那么Mysql中如何实现SQL中的ISNULL方法呢?

IFNULL( check_expression , replacement_value ),实现了SQL中的ISNULL方法。

见下sql语句

 代码如下 复制代码
mysql> SELECT IFNULL(1,0);
+-------------+
| IFNULL(1,0) |
+-------------+
|           1 |
+-------------+
1 row in set
 

由于 expr1 是 1,不为 NULL,所以函数返回 1. 我们可以再试下,如果让 expr1 为 NULL,是否能返回第二个参数呢? 不妨让 expr1 = 1/0, 由于除数为0,结果就为 NULL 了。

 代码如下 复制代码
mysql> SELECT IFNULL(1/0,'NowaMagic');
+-------------------------+
| IFNULL(1/0,'NowaMagic') |
+-------------------------+
| www.111cn.net               |
+-------------------------+
1 row in set
 

前面的参数为 NULL,就返回第二个参数 www.111cn.net 了。

这个函数不难理解,为什么要介绍这个函数呢?这是为了后面讲述 MySQL 下的高级黑客技术做的基础知识准备,你可以先试着理解这么一句 hack SQL

 代码如下 复制代码
UPDATE table SET views = '1' WHERE id = -2441 OR (ORD(MID((SELECT IFNULL(CAST(FirstName AS CHAR),0x20) FROM nowamagic.`tb2` ORDER BY id LIMIT 1,1),2,1))>112)#
 

 
Mysql的IN操作

见如下SQL语句

 代码如下 复制代码
select product
from ProductParameter
where val = 19116 OR val = 19127 OR val = 19128 OR val = 19119
 

OR了很多是不是觉得很繁琐呀?用IN就解决了:

 代码如下 复制代码
select product
from ProductParameter
where val in (19116, 19127, 19128, 19119))
 

更多详细内容请查看:http://www.111cn.net/database/mysql/57520.htm

bitsCN.com
Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
How does MySQL handle data replication?How does MySQL handle data replication?Apr 28, 2025 am 12:25 AM

MySQL processes data replication through three modes: asynchronous, semi-synchronous and group replication. 1) Asynchronous replication performance is high but data may be lost. 2) Semi-synchronous replication improves data security but increases latency. 3) Group replication supports multi-master replication and failover, suitable for high availability requirements.

How can you use the EXPLAIN statement to analyze query performance?How can you use the EXPLAIN statement to analyze query performance?Apr 28, 2025 am 12:24 AM

The EXPLAIN statement can be used to analyze and improve SQL query performance. 1. Execute the EXPLAIN statement to view the query plan. 2. Analyze the output results, pay attention to access type, index usage and JOIN order. 3. Create or adjust indexes based on the analysis results, optimize JOIN operations, and avoid full table scanning to improve query efficiency.

How do you back up and restore a MySQL database?How do you back up and restore a MySQL database?Apr 28, 2025 am 12:23 AM

Using mysqldump for logical backup and MySQLEnterpriseBackup for hot backup are effective ways to back up MySQL databases. 1. Use mysqldump to back up the database: mysqldump-uroot-pmydatabase>mydatabase_backup.sql. 2. Use MySQLEnterpriseBackup for hot backup: mysqlbackup--user=root-password=password--backup-dir=/path/to/backupbackup. When recovering, use the corresponding life

What are some common causes of slow queries in MySQL?What are some common causes of slow queries in MySQL?Apr 28, 2025 am 12:18 AM

The main reasons for slow MySQL query include missing or improper use of indexes, query complexity, excessive data volume and insufficient hardware resources. Optimization suggestions include: 1. Create appropriate indexes; 2. Optimize query statements; 3. Use table partitioning technology; 4. Appropriately upgrade hardware.

What are views in MySQL?What are views in MySQL?Apr 28, 2025 am 12:04 AM

MySQL view is a virtual table based on SQL query results and does not store data. 1) Views simplify complex queries, 2) Enhance data security, and 3) Maintain data consistency. Views are stored queries in databases that can be used like tables, but data is generated dynamically.

What are the differences in syntax between MySQL and other SQL dialects?What are the differences in syntax between MySQL and other SQL dialects?Apr 27, 2025 am 12:26 AM

MySQLdiffersfromotherSQLdialectsinsyntaxforLIMIT,auto-increment,stringcomparison,subqueries,andperformanceanalysis.1)MySQLusesLIMIT,whileSQLServerusesTOPandOracleusesROWNUM.2)MySQL'sAUTO_INCREMENTcontrastswithPostgreSQL'sSERIALandOracle'ssequenceandt

What is MySQL partitioning?What is MySQL partitioning?Apr 27, 2025 am 12:23 AM

MySQL partitioning improves performance and simplifies maintenance. 1) Divide large tables into small pieces by specific criteria (such as date ranges), 2) physically divide data into independent files, 3) MySQL can focus on related partitions when querying, 4) Query optimizer can skip unrelated partitions, 5) Choosing the right partition strategy and maintaining it regularly is key.

How do you grant and revoke privileges in MySQL?How do you grant and revoke privileges in MySQL?Apr 27, 2025 am 12:21 AM

How to grant and revoke permissions in MySQL? 1. Use the GRANT statement to grant permissions, such as GRANTALLPRIVILEGESONdatabase_name.TO'username'@'host'; 2. Use the REVOKE statement to revoke permissions, such as REVOKEALLPRIVILEGESONdatabase_name.FROM'username'@'host' to ensure timely communication of permission changes.

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Safe Exam Browser

Safe Exam Browser

Safe Exam Browser is a secure browser environment for taking online exams securely. This software turns any computer into a secure workstation. It controls access to any utility and prevents students from using unauthorized resources.

EditPlus Chinese cracked version

EditPlus Chinese cracked version

Small size, syntax highlighting, does not support code prompt function

SublimeText3 Linux new version

SublimeText3 Linux new version

SublimeText3 Linux latest version

SecLists

SecLists

SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.