I-database ye-MySQL iyisingatha kanjani imibuzo yenani elingelona i-null? I-MySQL ayisona isitatimende esikhethiwe

MySQL databaseUyisingatha kanjani imibuzo yenani elingelona ize?MySQL akusona isitatimende esikhethiwe

Ukuphatha Null ku-MySQL

Sesiyazi kakade ukuthi i-MySQL isebenzisa umyalo we-SQL KHETHA kanye nesigatshana esithi WHERE ukufunda idatha kuthebula ledatha, kodwa uma inkambu yesimo sombuzo enikeziwe ithi NULL, umyalo ungase ungasebenzi kahle.

Ukusingatha lesi simo, i-MySQL ihlinzeka ngama-opharetha amathathu amakhulu:

• AYINU:Lo opharetha ubuyisela iqiniso uma inani lekholomu ELINGU-NULL.
• AKUYONA NULL:Umsebenzisi ubuyisela iqiniso uma inani lekholomu lingelona LOKUPHAKATHI.
• <=>:  U-opharetha wesiqhathaniso (ngokungafani no-opharetha =) ubuyisela iqiniso lapho amanani amabili uma kuqhathaniswa ethi NULL.

Imisebenzi yokuqhathanisa enemibandela ku-NULL ikhethekile.Awukwazi ukusebenzisa = NULL noma ! =NULL ithola amanani angu-NULL kukholamu.

Ku-MySQL, ukuqhathaniswa kwevelu engu-NULL nanoma yiliphi elinye inani (ngisho no-NULL) njalo kubuyisela kungamanga, okungukuthi NULL = NULL ibuyisela okungamanga.

I-NULL iphathwa ku-MySQL kusetshenziswa i-IS NULL futhi AKUYONA opharetha ABAYINULL.

Sebenzisa inani elingu-NULL kumyalo womyalo

Esibonelweni esilandelayo, ithebula elithi chenweiliang_test_tbl kusizindalwazi esithi chenweiliang lisethwe ukuthi liqukathe amakholomu amabili, i-chenweiliang_author kanye ne-chenweiliang_count, futhi amanani angu-NULL asethwe ukuthi afakwe ku-chenweiliang_count.

Isibonelo

Zama izibonelo ezilandelayo:

root @ host ＃mysql -u root -p password; 输入密码：*******
 mysql > 使用chenweiliang ;
数据库改变了mysql > create table chenweiliang_test_tbl 
 - > （
 - > chenweiliang_author varchar （40 ）NOT NULL ， - > chenweiliang_count INT 
 - > ）;
查询OK ，0 行受影响（0.05 秒）mysql >
 
 
INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （' chenweiliang ' ，20 ）;
mysql > INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （' 陈沩亮博客' ，NULL ）;
mysql > INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （ ' Google ' ，NULL ）;
mysql > INSERT INTO chenweiliang_test_tbl （chenweiliang_author ，chenweiliang_count ）values （' FK ' ，20 ）;
 
mysql > SELECT * from chenweiliang_test_tbl ; + --------------- + -------------- + | chenweiliang_author | chenweiliang_count | + --------------- + -------------- + | chenweiliang | 20 | | 陈沩亮博客| NULL | | Google | NULL | | FK | 20 | + --------------- + -------------- +
 4 行中集合（0.01 秒） 

Esibonelweni esilandelayo ungabona = futhi ! I-opharetha = ayisebenzi:

mysql > SELECT * FROM chenweiliang_test_tbl WHERE chenweiliang_count = NULL ;
空集（0.00 秒）mysql > SELECT * FROM chenweiliang_test_tbl WHERE chenweiliang_count ！= NULL ;
空集（0.01 秒）

Ukuze uthole ukuthi ingabe ikholomu ye-chenweiliang_test_tbl kuthebula ledatha ingu-NULL, kufanele usebenziseI-NULLKanyeAKUKHO NULL, isibonelo esilandelayo:

mysql > SELECT * FROM chenweiliang_test_tbl WHERE chenweiliang_count IS NULL ; + --------------- + -------------- + | chenweiliang_author | chenweiliang_count | + --------------- + -------------- + | 陈沩亮博客| NULL | | Google | NULL | + --------------- + -------------- +
 2 行中的组（0.01 秒）的MySQL > SELECT * 从chenweiliang_test_tbl WHERE chenweiliang_count IS NOT 空值 
 
 ; + --------------- + -------------- + | chenweiliang_author | chenweiliang_count | + --------------- + -------------- + | chenweiliang | 20 | | FK | 20 | + --------------- + -------------- +
 2 行中的组（0.01 秒） 

Ukuphatha amanani angu-NULL ngombhalo we-PHP

Kuskripthi se-PHP, ungasebenzisa okuthi uma...okunye ukucubungula ukuthi okuguquguqukayo akunalutho futhi ukhiqize isitatimende esinemibandela esihambisanayo.

Esibonelweni esilandelayo i-PHP isetha okuguquguqukayo kwe-$chenweiliang_count bese isebenzisa lokho kuguquguquka ukuze iqhathanise nenkambu ye-chenweiliang_count kuthebula ledatha:

<?
php $ dbhost = ' localhost：3306 ' ; // mysql服务器主机地址

$ dbuser = ' root ' ; // mysql用户名
$ dbpass = ' 123456 ' ; // mysql用户名密码
$ conn = mysqli_connect （$ dbhost ，$ dbuser ，$ dbpass ）;
如果（！$ conn ）{ die （' 连接失败：' 。mysqli_error （$ conn ））;
} // 设置编码，防止中文乱码

mysqli_query （$ conn ，“ set names utf8 ” ）;
 
if （isset （$ chenweiliang_count ））{ $ sql = “ SELECT chenweiliang_author，chenweiliang_count FROM chenweiliang_test_tbl WHER chenweiliang_count = $ chenweiliang_count ” ;
} else { $ sql = “ SELECT chenweiliang_author，chenweiliang_count FROM chenweiliang_test_tbl WHER chenweiliang_count IS NULL ” ;
} mysqli_select_db （$ conn ，'


 chenweiliang ' ）;
$ retval = mysqli_query （$ conn ，$ sql ）;
if （！$ retval ）{ die （' 无法读取数据：' 。mysqli_error （$ conn ））;
} echo ' <h2>陈沩亮博客IS NULL测试<h2> ' ;
echo ' <table border =“1”> <tr> <td>作者</ td> <td>登陆次数</ td> </ tr> ' ;

 $ retval ，MYSQL_ASSOC ））{ echo “ <tr> ” 。
 “ <td> {$ row ['chenweiliang_author']} </ td> ” 。
 “ <td> {$ row ['chenweiliang_count']} </ td> ” 。
 “ </ tr> ” ;
} echo ' </ table> ' ;
mysqli_close （$ conn ）;
?>