Ingaba idatabase yeMySQL iyisingatha njani imibuzo yexabiso engeyo-null? I-MySQL ayiyongxelo ekhethiweyo

MySQL databaseUwusingatha njani umbuzo wexabiso ongeyo-null?MySQL ayisiyongxelo yokukhetha ililize

MySQL null nokuphathwa kwexabiso

Sele sisazi ukuba i-MySQL isebenzisa umyalelo we-SQL KHETHA kunye NAPHO igatya lokufunda idatha ukusuka kwitheyibhile yedatha, kodwa xa ummandla wombuzo wombuzo onikeziweyo u-NULL, umyalelo awunakusebenza ngokufanelekileyo.

Ukujongana nale meko, iMySQL ibonelela ngabaqhubi abakhulu abathathu:

• AYIKHO:Lo msebenzisi ubuyisela eyinyani xa ixabiso lekholamu YI-NULL.
• AKUKHO NULL:Xa ixabiso lekholamu lingeyiyo i-NULL, umsebenzisi ubuyisela inyaniso.
• <=>:  Umsebenzisi wothelekiso (ohlukileyo ku- = umqhubi), ubuyisela eyinyani xa amaxabiso amabini ethelekiswe AYI-NULL.

Umsebenzi wothelekiso olunemiqathango kwi-NULL ukhethekile. Awukwazi ukusebenzisa = NULL okanye ! =NULL ifumana NULL amaxabiso kumhlathi.

Kwi-MySQL, uthelekiso lwexabiso le-NULL nalo naliphi na elinye ixabiso (nokuba NULL) lihlala libuyisela okungeyonyani, oko kukuthi, NULL = NULL ibuyisela bubuxoki.

I-MySQL iphatha i-NULL isebenzisa i-IS NULL kwaye AYIKHO nabaqhubi be-NULL.

Ukusebenzisa ixabiso le-NULL kwi-prompt yomyalelo

Kulo mzekelo ulandelayo, itheyibhile chenweiliang_test_tbl kugcino lwedatha chenweiliang isetelwe ukuba iqulathe iikholamu ezimbini, i-chenweiliang_author kunye ne-chenweiliang_count, kwaye ixabiso le-NULL limiselwe ukuba lifakwe kwi-chenweiliang_count.

Umzekelo

Zama le mizekelo ilandelayo:

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） 

Kule mizekelo ilandelayo ungabona = kwaye ! Umsebenzisi = akanasiphumo:

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）

Ukufumana ukuba ikholamu ye-chenweiliang_test_tbl kwitheyibhile yedatha yiNULL, kufuneka usebenziseYI-NULLKunye kunyeA NGI NULL, le mizekelo ilandelayo:

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 amaxabiso a-NULL usebenzisa iskripthi se-PHP

Kwimibhalo ye-PHP, ungasebenzisa ukuba...enye ingxelo ukucubungula ukuba ukuguquguquka akunanto kwaye kuvelise iingxelo ezingqinelanayo ezinemiqathango.

Kulo mzekelo ulandelayo, i-PHP imisela i-$chenweiliang_count variable, ize isebenzise oku kuguquguquka ukuthelekisa nendawo ye-chenweiliang_count kwitheyibhile yedatha:

<?
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 ）;
?>