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MySQL databaseE fa'afefea ona fa'atautaia fesili e le fa'aleaogaina?MySQL e le'o le null select statement
Null taulimaina i MySQL
Ua uma ona matou iloa o loʻo faʻaaogaina e MySQL le SQL SELECT command ma le WHERE fuaiupu e faitau ai faʻamatalaga i le laulau faʻamaumauga, ae a oʻo i le NULL le tulaga ole fesili, atonu e le lelei le faʻatonuga.
Ina ia taulimaina lenei tulaga, e tuʻuina atu e MySQL ni faʻalapotopotoga tetele se tolu:
- E NULL:E fa'afo'i sa'o le tagata fa'atino pe a NULL le tau o le koluma.
- E LE NULL:E fa'afo'i sa'o le tagata fa'afoe pe a le NULL le tau o le koluma.
- <=>: O le faʻatusatusaga faʻapitoa (e le pei o le = operator) e toe foʻi moni pe a faʻatusatusa tau e lua e NULL.
Fa'atonuga fa'atusa i luga ole NULL e fa'apitoa.E le mafai ona e faʻaogaina = NULL poʻo ! =NULL maua NULL tau i le koluma.
I MySQL, faʻatusatusaina o se tau NULL ma soʻo se isi tau (e oʻo lava i le NULL) e toe foʻi mai sese, ie NULL = NULL toe foʻi sese.
NULL o loʻo faʻaaogaina i MySQL e faʻaaoga ai le IS NULL ma IS NOT NULL operators.
Fa'aaoga le tau NULL ile fa'atonuga
I le faʻataʻitaʻiga o loʻo mulimuli mai, o le laulau chenweiliang_test_tbl i le database chenweiliang ua setiina e aofia ai ni koluma se lua, chenweiliang_author ma chenweiliang_count, ma NULL tau e faʻapipiʻi i le chenweiliang_count.
Fa'ata'ita'iga
Fa'ata'ita'i fa'ata'ita'iga nei:
Fausia le laulau fa'amaumauga 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)
I le faʻataʻitaʻiga o loʻo i lalo e mafai ona e vaʻai = ma ! E le galue le = 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)
Ina ia iloa pe o le chenweiliang_test_tbl koluma i le laulau faʻamaumauga o le NULL, e tatau ona e faʻaogainaIS NULLmaE LE NULL, le fa'ata'ita'iga lea:
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)
Taulimaina NULL tau faʻatasi ma le PHP script
I le PHP script, e mafai ona e faʻaogaina le if...else faʻamatalaga e faʻatautaia pe leai se mea e fesuiaʻi ma maua ai se faʻamatalaga tuʻufaʻatasiga tutusa.
I le faʻataʻitaʻiga o loʻo mulimuli mai PHP seti le $ chenweiliang_count fesuiaiga ona faʻaaogaina lea o le fesuiaiga e faʻatusatusa i le chenweiliang_count fanua i le laulau faʻamatalaga:
MySQL ORDER BY suʻega:
<? 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 ); ?>
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