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MySQL databaseBawo ni lati mu awọn ibeere iye ti kii ṣe asan?MySQL ni ko asan yan gbólóhùn
Asan mu ni MySQL
A ti mọ tẹlẹ pe MySQL nlo aṣẹ SQL SELECT ati gbolohun NIBI lati ka data ninu tabili data, ṣugbọn nigbati aaye ipo ibeere ti a pese jẹ NULL, aṣẹ le ma ṣiṣẹ daradara.
Lati mu ipo yii, MySQL pese awọn oniṣẹ pataki mẹta:
- ODO NI:Oṣiṣẹ yii yoo pada ni otitọ nigbati iye ọwọn jẹ NULL.
- KO NI ODO:Oniṣẹ naa yoo pada ni otitọ nigbati iye ọwọn ko jẹ NULL.
- <=>: Oṣiṣẹ lafiwe (ko dabi = oniṣẹ) pada ni otitọ nigbati awọn iye meji ti a fiwera jẹ NULL.
Awọn iṣẹ lafiwe ipo lori NULL jẹ pataki.O ko le lo = NULL tabi ! = NULL wa awọn iye NULL ninu iwe naa.
Ni MySQL, lafiwe ti iye NULL pẹlu eyikeyi iye miiran (paapaa NULL) nigbagbogbo n pada eke, ie NULL = NULL pada eke.
NULL ni a mu ni MySQL ni lilo IS NULL ati kii ṣe awọn oniṣẹ NULL.
Lo NULL iye ni pipaṣẹ tọ
Ninu apẹẹrẹ atẹle, tabili chenweiliang_test_tbl ninu aaye data chenweiliang ti ṣeto lati ni awọn ọwọn meji ninu, chenweiliang_author ati chenweiliang_count, ati awọn iye NULL ti ṣeto lati fi sii ni chenweiliang_count.
Apeere
Gbiyanju awọn apẹẹrẹ wọnyi:
Ṣẹda data tabili chenweiliang_test_tbl
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）
Ni awọn wọnyi apẹẹrẹ o ti le ri = ati! Oniṣẹ = oniṣẹ ko ṣiṣẹ:
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）
Lati wa boya iwe chenweiliang_test_tbl ninu tabili data jẹ NULL, o gbọdọ loWA NULL和KI NI NULL, apẹẹrẹ atẹle:
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）
Mimu awọn iye NULL pẹlu iwe afọwọkọ PHP
Ninu iwe afọwọkọ PHP, o le lo if...ọrọ miiran lati ṣe ilana boya oniyipada naa ṣofo ati ṣe agbekalẹ alaye asọye ibamu kan.
Ninu apẹẹrẹ atẹle PHP ṣeto oniyipada $chenweiliang_count ati lẹhinna lo oniyipada yẹn lati ṣe afiwe pẹlu aaye chenweiliang_count ninu tabili data:
Ibere MySQL NIPA idanwo:
<?
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 ）;
?>
Ireti Chen Weiliang Blog ( https://www.chenweiliang.com/ ) pín “Bawo ni database MySQL ṣe n ṣakoso awọn ibeere iye ti kii ṣe asan? MySQL kii ṣe alaye yiyan asan” lati ṣe iranlọwọ fun ọ.
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