×ðÁú¿­Ê±¹ÙÍøµÇ¼

mysqlÖÐfrom dualʲôÒâ˼

from dual ÔÚ mysql ÖÐÓÃÓÚ´Óµ¥ÐÐÐéÄâ±í dual ÖмìË÷Êý¾Ý£¬³£ÓÃÓÚ»ñÈ¡Ä¿½ñÈÕÆÚºÍʱ¼ä¡¢Êý¾Ý¿â°æ±¾£¬»ò×÷ΪÅÌÎÊÖеÄռλ·û»òÖ´ÐÐ ddl Óï¾ä¡£

MySQL ÖÐµÄ FROM DUAL

ÎÊÌ⣺FROM DUAL ÔÚ MySQL ÖÐÊÇʲôÒâ˼£¿

»Ø¸²£º

FROM DUAL ÔÚ MySQL ÖÐÓÃÓÚ´ÓÒ»¸ö°üÀ¨¼òµ¥ÐеÄÐéÄâ±í DUAL ÖмìË÷Êý¾Ý¡£Ëüͨ³£ÓÃÓÚÔÚûÓÐÏÖʵÊý¾Ý±íµÄÇéÐÎÏÂÖ´ÐÐÅÌÎÊ»ò»ñÈ¡ÔªÊý¾ÝÐÅÏ¢¡£

ÏêϸڹÊÍ£º

DUAL ±íÊÇÒ»¸öÌØÊâµÄÐéÄâ±í£¬Ëü²»°üÀ¨ÈκÎÊý¾Ý£¬µ«Ê¼ÖÕ±£´æÓÚÊý¾Ý¿âÖС£Ëü°üÀ¨Ò»ÐУ¬ÆäÁÐÃûΪ DUMMY¡£

ʹÓà FROM DUAL Óï¾ä´Ó DUAL ±íÖмìË÷Êý¾Ýʱ£¬Ëü½«·µ»Ø¸Ã¼òµ¥ÐеÄÖµ£¬¼´ DUMMY¡£¸Ãֵͨ³£ÓÃÓÚ×÷Ϊռλ·û»ò³£Á¿£¬ÒÔ×ÊÖú½á¹¹ÅÌÎÊ¡£

FROM DUAL Óï¾äÓÐÒÔÏÂÓÃ;£º

»ñÈ¡Ä¿½ñÈÕÆÚºÍʱ¼ä£ºÊ¹Óà NOW() º¯Êý£¬ÀýÈç SELECT NOW() FROM DUAL¡£

»ñÈ¡Êý¾Ý¿â°æ±¾£ºÊ¹Óà VERSION() º¯Êý£¬ÀýÈç SELECT VERSION() FROM DUAL¡£

×÷Ϊ×ÓÅÌÎÊÖеÄռλ·û£ºÀýÈ磬Ëü¿ÉÒÔÓÃÓÚ»ñÈ¡¿ÕЧ¹û¼¯ÒÔÅþÁ¬µ½ÁíÒ»¸ö±í£¬ÀýÈç SELECT * FROM table1 WHERE id IN (SELECT DUMMY FROM DUAL)¡£

Ö´ÐÐ DDL Óï¾ä£ºÀýÈ磬Ëü¿ÉÒÔÓÃÓÚ½¨Éè»òɾ³ý±í£¬ÀýÈç CREATE TABLE table1 (id INT) FROM DUAL¡£

Óï·¨£º

SELECT <expression>
FROM DUAL;</expression>

µÇ¼ºó¸´ÖÆ

ÆäÖÐ ÊÇÒª·µ»ØµÄÖµ»òº¯Êý¡£

ÒÔÉϾÍÊÇmysqlÖÐfrom dualʲôÒâ˼µÄÏêϸÄÚÈÝ£¬¸ü¶àÇë¹Ø×¢±¾ÍøÄÚÆäËüÏà¹ØÎÄÕ£¡

ÃâÔð˵Ã÷£ºÒÔÉÏչʾÄÚÈÝȪԴÓÚÏàÖúýÌå¡¢ÆóÒµ»ú¹¹¡¢ÍøÓÑÌṩ»òÍøÂçÍøÂçÕûÀí£¬°æȨÕùÒéÓë±¾Õ¾Î޹أ¬ÎÄÕÂÉæ¼°¿´·¨Óë¿´·¨²»´ú±í×ðÁú¿­Ê±¹ÙÍøµÇ¼ÂËÓÍ»úÍø¹Ù·½Ì¬¶È£¬Çë¶ÁÕß½ö×ö²Î¿¼¡£±¾ÎĽӴýתÔØ£¬×ªÔØÇë˵Ã÷À´ÓÉ¡£ÈôÄúÒÔΪ±¾ÎÄÇÖÕ¼ÁËÄúµÄ°æȨÐÅÏ¢£¬»òÄú·¢Ã÷¸ÃÄÚÈÝÓÐÈκÎÉæ¼°ÓÐÎ¥¹«µÂ¡¢Ã°·¸Ö´·¨µÈÎ¥·¨ÐÅÏ¢£¬ÇëÄúÁ¬Ã¦ÁªÏµ×ðÁú¿­Ê±¹ÙÍøµÇ¼ʵʱÐÞÕý»òɾ³ý¡£

Ïà¹ØÐÂÎÅ

ÁªÏµ×ðÁú¿­Ê±¹ÙÍøµÇ¼

18523999891

¿É΢ÐÅÔÚÏß×Éѯ

ÊÂÇéʱ¼ä£ºÖÜÒ»ÖÁÖÜÎ壬9:30-18:30£¬½ÚãåÈÕÐÝÏ¢

QR code
ÍøÕ¾µØͼ