Kodi database ya MySQL imayankha bwanji mafunso osafunikira? MySQL si mawu osasankhidwa opanda pake

MySQL databaseMomwe mungayankhire mafunso opanda null value?MySQL si mawu osasankhidwa opanda pake

Kuwongolera zopanda pake mu MySQL

Tikudziwa kale kuti MySQL imagwiritsa ntchito lamulo la SQL SELECT ndi ndime ya WHERE kuti iwerenge deta mu tebulo la deta, koma pamene gawo lafunso lomwe laperekedwa ndi NULL, lamuloli silingagwire ntchito bwino.

Kuti athane ndi vutoli, MySQL imapereka othandizira atatu akuluakulu:

• NDI ZABWINO:Wogwiritsa ntchitoyu abwereranso zoona pamene chigawocho chili NULL.
• SI ZABWINO:Wogwiritsa ntchitoyo abwereranso moona pamene mtengo wagawo suli NULL.
• <=>:  Woyerekeza (mosiyana ndi = woyendetsa) amabwereranso pamene zikhalidwe ziwiri zofananizidwa ndi NULL.

Zoyeserera zofananira pa NULL ndizopadera.Simungagwiritse ntchito = NULL kapena ! = NULL imapeza NULL pamagawo.

Mu MySQL, kuyerekezera mtengo wa NULL ndi mtengo wina uliwonse (ngakhale NULL) nthawi zonse amabwerera zabodza, mwachitsanzo, NULL = NULL amabwerera zabodza.

NULL imayendetsedwa mu MySQL pogwiritsa ntchito IS NULL ndipo IS NOT NULL operators.

Gwiritsani ntchito mtengo wa NULL potsatira malangizo

Muchitsanzo chotsatirachi, tebulo chenweiliang_test_tbl mu nkhokwe ya chenweiliang yakhazikitsidwa kuti ikhale ndi mizere iwiri, chenweiliang_author ndi chenweiliang_count, ndipo NULL values ​​imayikidwa kuti iyikidwe mu chenweiliang_count.

Chitsanzo

Yesani zitsanzo zotsatirazi:

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） 

Muchitsanzo chotsatirachi mutha kuwona = ndi ! Wothandizira = sakugwira ntchito:

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）

Kuti mudziwe ngati gawo la chenweiliang_test_tbl pa tebulo la data ndi NULL, muyenera kugwiritsa ntchitoNDI NULLndipoSIYENERA, chitsanzo chotsatira:

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） 

Kusamalira ma NULL ndi PHP script

Mu PHP script, mungagwiritse ntchito if...else statement kuti muwone ngati kusinthaku kulibe kanthu ndi kupanga mawu ogwirizana nawo.

Muchitsanzo chotsatira PHP imayika $chenweiliang_count kusinthika kenako imagwiritsa ntchito kusinthaku kufananiza ndi gawo la chenweiliang_count pa tebulo la data:

<?
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 ）;
?>