Pehea e mālama ai ka waihona MySQL i nā nīnau waiwai ʻole? ʻAʻole ʻo MySQL kahi ʻōlelo koho koho

MySQL waihonaPehea e mālama ai i nā nīnau waiwai ʻole?MySQL ʻaʻole ia ka ʻōlelo koho null

ʻO ka lawelawe ʻana ma MySQL

Ua ʻike mua mākou e hoʻohana ana ʻo MySQL i ke kauoha SQL SELECT a me ka māhele WHERE e heluhelu i ka ʻikepili i ka papa ʻikepili, akā inā ʻo NULL ke kūlana nīnau i hāʻawi ʻia, ʻaʻole pono ke kauoha.

No ka mālama ʻana i kēia kūlana, hāʻawi ʻo MySQL i ʻekolu mau mea hana nui:

• IS NULL:Hoʻihoʻi mai kēia mea hoʻohana i ka ʻoiaʻiʻo inā NULL ke kolamu.
• ʻAʻole NULL:Hoʻihoʻi ka mea hoʻohana i ka ʻoiaʻiʻo inā ʻaʻole NULL ke kolamu.
• <=>:  ʻO ka mea hoʻohālikelike hoʻohālikelike (ʻaʻole like me ka = operator) e hoʻi i ka ʻoiaʻiʻo i ka wā i hoʻohālikelike ʻia nā waiwai ʻelua he NULL.

He kūikawā nā hana hoʻohālikelike kūlana ma NULL.ʻAʻole hiki iā ʻoe ke hoʻohana = NULL a i ʻole! = NULL loaʻa nā waiwai NULL i ke kolamu.

Ma MySQL, hoʻihoʻi hewa ka hoʻohālikelike ʻana i kahi waiwai NULL me kekahi waiwai ʻē aʻe (ʻo ia hoʻi NULL), ʻo ia hoʻi NULL = NULL hoʻihoʻi wahaheʻe.

Hoʻohana ʻia ʻo NULL ma MySQL me ka hoʻohana ʻana i ka IS NULL a ʻaʻole ʻo IS NULL.

E hoʻohana i ka waiwai NULL ma ke kauoha kauoha

Ma kēia laʻana, ua hoʻonohonoho ʻia ka papa ʻaina chenweiliang_test_tbl i ka waihona chenweiliang i ʻelua kolamu, chenweiliang_author a me chenweiliang_count, a ua hoʻonohonoho ʻia nā waiwai NULL e hoʻokomo i loko o chenweiliang_count.

laʻana

E ho'āʻo i kēia mau laʻana:

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） 

Ma kēia hiʻohiʻona hiki iā ʻoe ke ʻike = a! ʻAʻole hana ka = operator:

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）

No ka ʻike inā he NULL ka kolamu chenweiliang_test_tbl i ka papa ʻikepili, pono ʻoe e hoʻohana.ʻO NULLaNO KA NULL, ka laʻana penei:

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） 

Ka mālama ʻana i nā waiwai NULL me ka palapala PHP

Ma ka palapala PHP, hiki iā ʻoe ke hoʻohana i ka ʻōlelo inā...ē aʻe e hoʻoponopono ai inā he kaʻawale ka hoʻololi a hana i kahi ʻōlelo kūlana pili.

Ma kēia hiʻohiʻona PHP e hoʻonohonoho i ka $chenweiliang_count variable a laila hoʻohana i kēlā ʻano hoʻololi e hoʻohālikelike me ka chenweiliang_count kahua ma ka papa ʻikepili:

<?
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 ）;
?>