Sidee buu xogta MySQL u maareeyaa weydiimaha aan qiimaha lahayn? MySQL ma aha hadal doorasho bura

database MySQLSidee wax looga qabtaa weydiimaha aan qiimaha lahayn?MySQL ma aha hadal been ah

Wax ka qabashada aan jirin ee MySQL

Waxaan horeyba u ognahay in MySQL ay isticmaasho amarka SQL SELECT iyo qeebta WHERE si loo akhriyo xogta ku jirta shaxda xogta, laakiin marka goobta xaalada su'aaluhu ay tahay NULL, amarku si sax ah uma shaqeeyo.

Si loo maareeyo xaaladdan, MySQL waxay bixisaa saddex hawl wadeen oo waaweyn:

• WAA NULL:Hawlwadeenkani waxa uu soo noqdaa runta marka qiimihii tiirka uu yahay NULL.
• BAL MA AHA:Hawlwadeenku waxa uu soo noqdaa runta marka qiimihii tiirka aanu NULL ahayn.
• <=>:  Hawlwadeenka isbarbardhigga (ka duwan kan = hawlwadeenka) wuxuu soo noqdaa run marka labada qiyam ee la barbar dhigo NULL.

Hawlgallada isbarbardhigga shuruudaha ah ee NULL waa kuwo gaar ah.Ma isticmaali kartid = NULL ama ! =NULL waxay ka helaysaa qiyamka NULL ee tiirka.

MySQL dhexdeeda, isbarbardhigga qiimaha NULL iyo qiime kasta oo kale (xitaa NULL) had iyo jeer been buu soo noqdaa, ie NULL = NULL been buu soo noqdaa.

NULL waxaa lagu maamulaa MySQL iyadoo la isticmaalayo IS NULL oo AAN NULL ahayn hawlwadeenada

Isticmaal qiimaha NULL si degdeg ah amarka

Tusaalaha soo socda, miiska chenweiliang_test_tbl ee xogta xogta chenweiliang waxa loo dejiyay inuu ka kooban yahay laba tiir, chenweiliang_author iyo chenweiliang_count, iyo NULL qiyamka ayaa lagu dajiyay in la geliyo chenweiliang_count.

tusaale

Isku day tusaalooyinka soo socda:

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） 

Tusaalaha soo socda waxaad arki kartaa = iyo ! Shaqaalaha = ma shaqeeyo:

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）

Si aad u ogaato in tiirka chenweiliang_test_tbl ee shaxda xogta uu yahay NULL, waa inaad isticmaashaaWAA DHAMMAANOoWAA MALAHAN, tusaale ahaan:

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） 

Ku maaraynta qiyamka NULL ee qoraalka PHP

Farta PHP, waxaad isticmaali kartaa if...kale bayaan si aad uga baaraandegto in doorsoomuhu faaruq yahay iyo in uu dhaliyo bayaan shuruudaysan.

Tusaalaha soo socda PHP wuxuu dejiyaa doorsoomaha $chenweiliang_count ka dibna wuxuu isticmaalaa doorsoomahaas si uu ula barbar dhigo goobta chenweiliang_count ee shaxda xogta:

<?
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 ）;
?>